Add IPC config subcommand

This patch adds a new mechanism for changing configuration options
without editing the configuration file, by sending options to running
instances through `alacritty msg`.

Each window will load Alacritty's configuration file by default and then
accept IPC messages for config updates using the `alacritty msg config`
subcommand. By default all windows will be updated, individual windows
can be addressed using `alacritty msg config --window-id
"$ALACRITTY_WINDOW_ID"`.

Each option will replace the config's current value and cannot be reset
until Alacritty is restarted or the option is overwritten with a new
value.

Configuration options are passed in the format `field.subfield=value`,
where `value` is interpreted as yaml.

Closes #472.

32 files changed, 630 insertions, 165 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 06c70622..c786c9a4 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -18,6 +18,7 @@ checksum = "aae1277d39aeec15cb388266ecc24b11c80469deae6067e17a1a7aa9e5c1f234"
 name = "alacritty"
 version = "0.11.0-dev"
 dependencies = [
+ "alacritty_config",
  "alacritty_config_derive",
  "alacritty_terminal",
  "bitflags",
@@ -50,9 +51,19 @@ dependencies = [
 ]
 
 [[package]]
+name = "alacritty_config"
+version = "0.1.0"
+dependencies = [
+ "log",
+ "serde",
+ "serde_yaml",
+]
+
+[[package]]
 name = "alacritty_config_derive"
 version = "0.1.0"
 dependencies = [
+ "alacritty_config",
  "log",
  "proc-macro2",
  "quote",
@@ -65,6 +76,7 @@ dependencies = [
 name = "alacritty_terminal"
 version = "0.17.0-dev"
 dependencies = [
+ "alacritty_config",
  "alacritty_config_derive",
  "base64",
  "bitflags",
diff --git a/Cargo.toml b/Cargo.toml
index 7a6dec80..aad21557 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,6 +2,7 @@
 members = [
     "alacritty",
     "alacritty_terminal",
+    "alacritty_config",
     "alacritty_config_derive",
 ]
 
diff --git a/alacritty/Cargo.toml b/alacritty/Cargo.toml
index 989f0a83..9e858d70 100644
--- a/alacritty/Cargo.toml
+++ b/alacritty/Cargo.toml
@@ -18,8 +18,12 @@ default-features = false
 path = "../alacritty_config_derive"
 version = "0.1.0"
 
+[dependencies.alacritty_config]
+path = "../alacritty_config"
+version = "0.1.0"
+
 [dependencies]
-clap = { version = "3.0.0", features = ["derive"] }
+clap = { version = "3.0.0", features = ["derive", "env"] }
 log = { version = "0.4", features = ["std", "serde"] }
 fnv = "1"
 serde = { version = "1", features = ["derive"] }
diff --git a/alacritty/src/cli.rs b/alacritty/src/cli.rs
index e9c563d4..e7aae207 100644
--- a/alacritty/src/cli.rs
+++ b/alacritty/src/cli.rs
@@ -81,14 +81,7 @@ impl Options {
         let mut options = Self::parse();
 
         // Convert `--option` flags into serde `Value`.
-        for option in &options.option {
-            match option_as_value(option) {
-                Ok(value) => {
-                    options.config_options = serde_utils::merge(options.config_options, value);
-                },
-                Err(_) => eprintln!("Invalid CLI config option: {:?}", option),
-            }
-        }
+        options.config_options = options_as_value(&options.option);
 
         options
     }
@@ -132,7 +125,18 @@ impl Options {
     }
 }
 
-/// Format an option in the format of `parent.field=value` to a serde Value.
+/// Combine multiple options into a [`serde_yaml::Value`].
+pub fn options_as_value(options: &[String]) -> Value {
+    options.iter().fold(Value::default(), |value, option| match option_as_value(option) {
+        Ok(new_value) => serde_utils::merge(value, new_value),
+        Err(_) => {
+            eprintln!("Ignoring invalid option: {:?}", option);
+            value
+        },
+    })
+}
+
+/// Parse an option in the format of `parent.field=value` as a serde Value.
 fn option_as_value(option: &str) -> Result<Value, serde_yaml::Error> {
     let mut yaml_text = String::with_capacity(option.len());
     let mut closing_brackets = String::new();
@@ -266,7 +270,7 @@ pub enum Subcommands {
 #[derive(Args, Debug)]
 pub struct MessageOptions {
     /// IPC socket connection path override.
-    #[clap(long, short, value_hint = ValueHint::FilePath)]
+    #[clap(short, long, value_hint = ValueHint::FilePath)]
     pub socket: Option<PathBuf>,
 
     /// Message which should be sent.
@@ -280,9 +284,12 @@ pub struct MessageOptions {
 pub enum SocketMessage {
     /// Create a new window in the same Alacritty process.
     CreateWindow(WindowOptions),
+
+    /// Update the Alacritty configuration.
+    Config(IpcConfig),
 }
 
-/// Subset of options that we pass to a 'create-window' subcommand.
+/// Subset of options that we pass to 'create-window' IPC subcommand.
 #[derive(Serialize, Deserialize, Args, Default, Clone, Debug, PartialEq, Eq)]
 pub struct WindowOptions {
     /// Terminal options which can be passed via IPC.
@@ -294,6 +301,25 @@ pub struct WindowOptions {
     pub window_identity: WindowIdentity,
 }
 
+/// Parameters to the `config` IPC subcommand.
+#[cfg(unix)]
+#[derive(Args, Serialize, Deserialize, Default, Debug, Clone, PartialEq, Eq)]
+pub struct IpcConfig {
+    /// Configuration file options [example: cursor.style=Beam].
+    #[clap(required = true, value_name = "CONFIG_OPTIONS")]
+    pub options: Vec<String>,
+
+    /// Window ID for the new config.
+    ///
+    /// Use `-1` to apply this change to all windows.
+    #[clap(short, long, allow_hyphen_values = true, env = "ALACRITTY_WINDOW_ID")]
+    pub window_id: Option<i128>,
+
+    /// Clear all runtime configuration changes.
+    #[clap(short, long, conflicts_with = "options")]
+    pub reset: bool,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/alacritty/src/config/bindings.rs b/alacritty/src/config/bindings.rs
index c48b2d75..72ea88b2 100644
--- a/alacritty/src/config/bindings.rs
+++ b/alacritty/src/config/bindings.rs
@@ -9,7 +9,7 @@ use serde::de::{self, Error as SerdeError, MapAccess, Unexpected, Visitor};
 use serde::{Deserialize, Deserializer};
 use serde_yaml::Value as SerdeValue;
 
-use alacritty_config_derive::ConfigDeserialize;
+use alacritty_config_derive::{ConfigDeserialize, SerdeReplace};
 
 use alacritty_terminal::config::Program;
 use alacritty_terminal::term::TermMode;
@@ -1191,7 +1191,7 @@ impl<'a> Deserialize<'a> for KeyBinding {
 ///
 /// Our deserialize impl wouldn't be covered by a derive(Deserialize); see the
 /// impl below.
-#[derive(Debug, Copy, Clone, Hash, Default, Eq, PartialEq)]
+#[derive(SerdeReplace, Debug, Copy, Clone, Hash, Default, Eq, PartialEq)]
 pub struct ModsWrapper(pub ModifiersState);
 
 impl ModsWrapper {
diff --git a/alacritty/src/config/font.rs b/alacritty/src/config/font.rs
index d3431171..9c431b15 100644
--- a/alacritty/src/config/font.rs
+++ b/alacritty/src/config/font.rs
@@ -4,7 +4,7 @@ use crossfont::Size as FontSize;
 use serde::de::{self, Visitor};
 use serde::{Deserialize, Deserializer};
 
-use alacritty_config_derive::ConfigDeserialize;
+use alacritty_config_derive::{ConfigDeserialize, SerdeReplace};
 
 use crate::config::ui_config::Delta;
 
@@ -129,7 +129,7 @@ impl SecondaryFontDescription {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(SerdeReplace, Debug, Clone, PartialEq, Eq)]
 struct Size(FontSize);
 
 impl Default for Size {
diff --git a/alacritty/src/config/ui_config.rs b/alacritty/src/config/ui_config.rs
index 28139d27..a332b737 100644
--- a/alacritty/src/config/ui_config.rs
+++ b/alacritty/src/config/ui_config.rs
@@ -9,7 +9,7 @@ use serde::de::{Error as SerdeError, MapAccess, Visitor};
 use serde::{self, Deserialize, Deserializer};
 use unicode_width::UnicodeWidthChar;
 
-use alacritty_config_derive::ConfigDeserialize;
+use alacritty_config_derive::{ConfigDeserialize, SerdeReplace};
 use alacritty_terminal::config::{
     Config as TerminalConfig, Percentage, Program, LOG_TARGET_CONFIG,
 };
@@ -30,7 +30,7 @@ use crate::config::window::WindowConfig;
 const URL_REGEX: &str = "(ipfs:|ipns:|magnet:|mailto:|gemini:|gopher:|https:|http:|news:|file:|git:|ssh:|ftp:)\
                          [^\u{0000}-\u{001F}\u{007F}-\u{009F}<>\"\\s{-}\\^⟨⟩`]+";
 
-#[derive(ConfigDeserialize, Debug, PartialEq)]
+#[derive(ConfigDeserialize, Clone, Debug, PartialEq)]
 pub struct UiConfig {
     /// Font configuration.
     pub font: Font,
@@ -145,7 +145,7 @@ impl UiConfig {
     }
 }
 
-#[derive(Debug, PartialEq, Eq)]
+#[derive(SerdeReplace, Clone, Debug, PartialEq, Eq)]
 struct KeyBindings(Vec<KeyBinding>);
 
 impl Default for KeyBindings {
@@ -163,7 +163,7 @@ impl<'de> Deserialize<'de> for KeyBindings {
     }
 }
 
-#[derive(Debug, PartialEq, Eq)]
+#[derive(SerdeReplace, Clone, Debug, PartialEq, Eq)]
 struct MouseBindings(Vec<MouseBinding>);
 
 impl Default for MouseBindings {
@@ -223,7 +223,7 @@ pub struct Delta<T: Default> {
 }
 
 /// Regex terminal hints.
-#[derive(ConfigDeserialize, Debug, PartialEq, Eq)]
+#[derive(ConfigDeserialize, Clone, Debug, PartialEq, Eq)]
 pub struct Hints {
     /// Characters for the hint labels.
     alphabet: HintsAlphabet,
@@ -273,7 +273,7 @@ impl Hints {
     }
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(SerdeReplace, Clone, Debug, PartialEq, Eq)]
 struct HintsAlphabet(String);
 
 impl Default for HintsAlphabet {
diff --git a/alacritty/src/config/window.rs b/alacritty/src/config/window.rs
index 80df87b7..5d63d60f 100644
--- a/alacritty/src/config/window.rs
+++ b/alacritty/src/config/window.rs
@@ -6,7 +6,7 @@ use log::{error, warn};
 use serde::de::{self, MapAccess, Visitor};
 use serde::{Deserialize, Deserializer, Serialize};
 
-use alacritty_config_derive::ConfigDeserialize;
+use alacritty_config_derive::{ConfigDeserialize, SerdeReplace};
 use alacritty_terminal::config::{Percentage, LOG_TARGET_CONFIG};
 use alacritty_terminal::index::Column;
 
@@ -201,7 +201,7 @@ pub struct Dimensions {
 }
 
 /// Window class hint.
-#[derive(Serialize, Debug, Clone, PartialEq, Eq)]
+#[derive(SerdeReplace, Serialize, Debug, Clone, PartialEq, Eq)]
 pub struct Class {
     pub general: String,
     pub instance: String,
diff --git a/alacritty/src/display/window.rs b/alacritty/src/display/window.rs
index eac12a22..95b42345 100644
--- a/alacritty/src/display/window.rs
+++ b/alacritty/src/display/window.rs
@@ -405,16 +405,6 @@ impl Window {
         self.window().request_user_attention(attention);
     }
 
-    #[cfg(all(feature = "x11", not(any(target_os = "macos", windows))))]
-    pub fn x11_window_id(&self) -> Option<usize> {
-        self.window().xlib_window().map(|xlib_window| xlib_window as usize)
-    }
-
-    #[cfg(any(not(feature = "x11"), target_os = "macos", windows))]
-    pub fn x11_window_id(&self) -> Option<usize> {
-        None
-    }
-
     pub fn id(&self) -> WindowId {
         self.window().id()
     }
diff --git a/alacritty/src/event.rs b/alacritty/src/event.rs
index b4258a03..bd4b60b2 100644
--- a/alacritty/src/event.rs
+++ b/alacritty/src/event.rs
@@ -9,6 +9,7 @@ use std::fmt::Debug;
 #[cfg(not(windows))]
 use std::os::unix::io::RawFd;
 use std::path::PathBuf;
+use std::rc::Rc;
 use std::time::{Duration, Instant};
 use std::{env, f32, mem};
 
@@ -38,6 +39,8 @@ use alacritty_terminal::selection::{Selection, SelectionType};
 use alacritty_terminal::term::search::{Match, RegexSearch};
 use alacritty_terminal::term::{self, ClipboardType, Term, TermMode};
 
+#[cfg(unix)]
+use crate::cli::IpcConfig;
 use crate::cli::{Options as CliOptions, WindowOptions};
 use crate::clipboard::Clipboard;
 use crate::config::ui_config::{HintAction, HintInternalAction};
@@ -93,6 +96,8 @@ pub enum EventType {
     Message(Message),
     Scroll(Scroll),
     CreateWindow(WindowOptions),
+    #[cfg(unix)]
+    IpcConfig(IpcConfig),
     BlinkCursor,
     BlinkCursorTimeout,
     SearchNext,
@@ -184,7 +189,7 @@ pub struct ActionContext<'a, N, T> {
     pub modifiers: &'a mut ModifiersState,
     pub display: &'a mut Display,
     pub message_buffer: &'a mut MessageBuffer,
-    pub config: &'a mut UiConfig,
+    pub config: &'a UiConfig,
     pub cursor_blink_timed_out: &'a mut bool,
     pub event_loop: &'a EventLoopWindowTarget<Event>,
     pub event_proxy: &'a EventLoopProxy<Event>,
@@ -1162,6 +1167,8 @@ impl input::Processor<EventProxy, ActionContext<'_, Notifier, EventProxy>> {
                     TerminalEvent::Exit => (),
                     TerminalEvent::CursorBlinkingChange => self.ctx.update_cursor_blinking(),
                 },
+                #[cfg(unix)]
+                EventType::IpcConfig(_) => (),
                 EventType::ConfigReload(_) | EventType::CreateWindow(_) => (),
             },
             GlutinEvent::RedrawRequested(_) => *self.ctx.dirty = true,
@@ -1292,7 +1299,7 @@ pub struct Processor {
     wayland_event_queue: Option<EventQueue>,
     windows: HashMap<WindowId, WindowContext>,
     cli_options: CliOptions,
-    config: UiConfig,
+    config: Rc<UiConfig>,
 }
 
 impl Processor {
@@ -1313,8 +1320,8 @@ impl Processor {
 
         Processor {
             windows: HashMap::new(),
+            config: Rc::new(config),
             cli_options,
-            config,
             #[cfg(all(feature = "wayland", not(any(target_os = "macos", windows))))]
             wayland_event_queue,
         }
@@ -1328,7 +1335,7 @@ impl Processor {
         options: WindowOptions,
     ) -> Result<(), Box<dyn Error>> {
         let window_context = WindowContext::new(
-            &self.config,
+            self.config.clone(),
             &options,
             event_loop,
             proxy,
@@ -1436,7 +1443,6 @@ impl Processor {
                         window_context.handle_event(
                             event_loop,
                             &proxy,
-                            &mut self.config,
                             &mut clipboard,
                             &mut scheduler,
                             GlutinEvent::RedrawEventsCleared,
@@ -1457,13 +1463,27 @@ impl Processor {
 
                     // Load config and update each terminal.
                     if let Ok(config) = config::reload(&path, &self.cli_options) {
-                        let old_config = mem::replace(&mut self.config, config);
+                        self.config = Rc::new(config);
 
                         for window_context in self.windows.values_mut() {
-                            window_context.update_config(&old_config, &self.config);
+                            window_context.update_config(self.config.clone());
                         }
                     }
                 },
+                // Process IPC config update.
+                #[cfg(unix)]
+                GlutinEvent::UserEvent(Event {
+                    payload: EventType::IpcConfig(ipc_config),
+                    window_id,
+                }) => {
+                    for (_, window_context) in self
+                        .windows
+                        .iter_mut()
+                        .filter(|(id, _)| window_id.is_none() || window_id == Some(**id))
+                    {
+                        window_context.update_ipc_config(self.config.clone(), ipc_config.clone());
+                    }
+                },
                 // Create a new terminal window.
                 GlutinEvent::UserEvent(Event {
                     payload: EventType::CreateWindow(options), ..
@@ -1485,7 +1505,6 @@ impl Processor {
                         window_context.handle_event(
                             event_loop,
                             &proxy,
-                            &mut self.config,
                             &mut clipboard,
                             &mut scheduler,
                             event.clone().into(),
@@ -1500,7 +1519,6 @@ impl Processor {
                         window_context.handle_event(
                             event_loop,
                             &proxy,
-                            &mut self.config,
                             &mut clipboard,
                             &mut scheduler,
                             event,
diff --git a/alacritty/src/ipc.rs b/alacritty/src/ipc.rs
index d4c807ba..e229a048 100644
--- a/alacritty/src/ipc.rs
+++ b/alacritty/src/ipc.rs
@@ -7,6 +7,7 @@ use std::path::PathBuf;
 use std::{env, fs, process};
 
 use glutin::event_loop::EventLoopProxy;
+use glutin::window::WindowId;
 use log::warn;
 
 use alacritty_terminal::thread;
@@ -62,6 +63,14 @@ pub fn spawn_ipc_socket(options: &Options, event_proxy: EventLoopProxy<Event>) -
                     let event = Event::new(EventType::CreateWindow(options), None);
                     let _ = event_proxy.send_event(event);
                 },
+                SocketMessage::Config(ipc_config) => {
+                    let window_id = ipc_config
+                        .window_id
+                        .and_then(|id| u64::try_from(id).ok())
+                        .map(WindowId::from);
+                    let event = Event::new(EventType::IpcConfig(ipc_config), window_id);
+                    let _ = event_proxy.send_event(event);
+                },
             }
         }
     });
diff --git a/alacritty/src/window_context.rs b/alacritty/src/window_context.rs
index e75f118d..9a2a8730 100644
--- a/alacritty/src/window_context.rs
+++ b/alacritty/src/window_context.rs
@@ -6,6 +6,7 @@ use std::io::Write;
 use std::mem;
 #[cfg(not(windows))]
 use std::os::unix::io::{AsRawFd, RawFd};
+use std::rc::Rc;
 #[cfg(not(any(target_os = "macos", windows)))]
 use std::sync::atomic::Ordering;
 use std::sync::Arc;
@@ -14,11 +15,12 @@ use crossfont::Size;
 use glutin::event::{Event as GlutinEvent, ModifiersState, WindowEvent};
 use glutin::event_loop::{EventLoopProxy, EventLoopWindowTarget};
 use glutin::window::WindowId;
-use log::info;
+use log::{error, info};
 use serde_json as json;
 #[cfg(all(feature = "wayland", not(any(target_os = "macos", windows))))]
 use wayland_client::EventQueue;
 
+use alacritty_config::SerdeReplace;
 use alacritty_terminal::event::Event as TerminalEvent;
 use alacritty_terminal::event_loop::{EventLoop as PtyEventLoop, Msg, Notifier};
 use alacritty_terminal::grid::{Dimensions, Scroll};
@@ -28,6 +30,8 @@ use alacritty_terminal::term::test::TermSize;
 use alacritty_terminal::term::{Term, TermMode};
 use alacritty_terminal::tty;
 
+#[cfg(unix)]
+use crate::cli::IpcConfig;
 use crate::cli::WindowOptions;
 use crate::clipboard::Clipboard;
 use crate::config::UiConfig;
@@ -58,12 +62,14 @@ pub struct WindowContext {
     master_fd: RawFd,
     #[cfg(not(windows))]
     shell_pid: u32,
+    ipc_config: Vec<(String, serde_yaml::Value)>,
+    config: Rc<UiConfig>,
 }
 
 impl WindowContext {
     /// Create a new terminal window context.
     pub fn new(
-        config: &UiConfig,
+        config: Rc<UiConfig>,
         options: &WindowOptions,
         window_event_loop: &EventLoopWindowTarget<Event>,
         proxy: EventLoopProxy<Event>,
@@ -81,7 +87,7 @@ impl WindowContext {
         //
         // The display manages a window and can draw the terminal.
         let display = Display::new(
-            config,
+            &config,
             window_event_loop,
             &identity,
             #[cfg(all(feature = "wayland", not(any(target_os = "macos", windows))))]
@@ -109,7 +115,7 @@ impl WindowContext {
         // The PTY forks a process to run the shell on the slave side of the
         // pseudoterminal. A file descriptor for the master side is retained for
         // reading/writing to the shell.
-        let pty = tty::new(&pty_config, display.size_info.into(), display.window.x11_window_id())?;
+        let pty = tty::new(&pty_config, display.size_info.into(), display.window.id().into())?;
 
         #[cfg(not(windows))]
         let master_fd = pty.file().as_raw_fd();
@@ -142,23 +148,27 @@ impl WindowContext {
             event_proxy.send_event(TerminalEvent::CursorBlinkingChange.into());
         }
 
+        let font_size = config.font.size();
+
         // Create context for the Alacritty window.
         Ok(WindowContext {
-            font_size: config.font.size(),
-            notifier: Notifier(loop_tx),
+            preserve_title,
+            font_size,
             terminal,
             display,
-            preserve_title,
             #[cfg(not(windows))]
             master_fd,
             #[cfg(not(windows))]
             shell_pid,
+            config,
+            notifier: Notifier(loop_tx),
             cursor_blink_timed_out: Default::default(),
             suppress_chars: Default::default(),
             message_buffer: Default::default(),
             received_count: Default::default(),
             search_state: Default::default(),
             event_queue: Default::default(),
+            ipc_config: Default::default(),
             modifiers: Default::default(),
             mouse: Default::default(),
             dirty: Default::default(),
@@ -167,33 +177,49 @@ impl WindowContext {
     }
 
     /// Update the terminal window to the latest config.
-    pub fn update_config(&mut self, old_config: &UiConfig, config: &UiConfig) {
-        self.display.update_config(config);
-        self.terminal.lock().update_config(&config.terminal_config);
+    pub fn update_config(&mut self, new_config: Rc<UiConfig>) {
+        let old_config = mem::replace(&mut self.config, new_config);
+
+        // Apply ipc config if there are overrides.
+        if !self.ipc_config.is_empty() {
+            let mut config = (*self.config).clone();
+
+            // Apply each option.
+            for (key, value) in &self.ipc_config {
+                if let Err(err) = config.replace(key, value.clone()) {
+                    error!("Unable to override option '{}': {}", key, err);
+                }
+            }
+
+            self.config = Rc::new(config);
+        }
+
+        self.display.update_config(&self.config);
+        self.terminal.lock().update_config(&self.config.terminal_config);
 
         // Reload cursor if its thickness has changed.
         if (old_config.terminal_config.cursor.thickness()
-            - config.terminal_config.cursor.thickness())
+            - self.config.terminal_config.cursor.thickness())
         .abs()
             > f32::EPSILON
         {
             self.display.pending_update.set_cursor_dirty();
         }
 
-        if old_config.font != config.font {
+        if old_config.font != self.config.font {
             // Do not update font size if it has been changed at runtime.
             if self.font_size == old_config.font.size() {
-                self.font_size = config.font.size();
+                self.font_size = self.config.font.size();
             }
 
-            let font = config.font.clone().with_size(self.font_size);
+            let font = self.config.font.clone().with_size(self.font_size);
             self.display.pending_update.set_font(font);
         }
 
         // Update display if padding options were changed.
         let window_config = &old_config.window;
-        if window_config.padding(1.) != config.window.padding(1.)
-            || window_config.dynamic_padding != config.window.dynamic_padding
+        if window_config.padding(1.) != self.config.window.padding(1.)
+            || window_config.dynamic_padding != self.config.window.dynamic_padding
         {
             self.display.pending_update.dirty = true;
         }
@@ -206,18 +232,18 @@ impl WindowContext {
         // │ N  │       Y       │              N              ││     N     │
         // │ N  │       N       │              _              ││     Y     │
         if !self.preserve_title
-            && (!config.window.dynamic_title
+            && (!self.config.window.dynamic_title
                 || self.display.window.title() == old_config.window.identity.title)
         {
-            self.display.window.set_title(config.window.identity.title.clone());
+            self.display.window.set_title(self.config.window.identity.title.clone());
         }
 
         // Disable shadows for transparent windows on macOS.
         #[cfg(target_os = "macos")]
-        self.display.window.set_has_shadow(config.window_opacity() >= 1.0);
+        self.display.window.set_has_shadow(self.config.window_opacity() >= 1.0);
 
         // Update hint keys.
-        self.display.hint_state.update_alphabet(config.hints.alphabet());
+        self.display.hint_state.update_alphabet(self.config.hints.alphabet());
 
         // Update cursor blinking.
         let event = Event::new(TerminalEvent::CursorBlinkingChange.into(), None);
@@ -226,12 +252,39 @@ impl WindowContext {
         self.dirty = true;
     }
 
+    /// Update the IPC config overrides.
+    #[cfg(unix)]
+    pub fn update_ipc_config(&mut self, config: Rc<UiConfig>, ipc_config: IpcConfig) {
+        self.ipc_config.clear();
+
+        if !ipc_config.reset {
+            for option in &ipc_config.options {
+                // Separate config key/value.
+                let (key, value) = match option.split_once('=') {
+                    Some(split) => split,
+                    None => {
+                        error!("'{}': IPC config option missing value", option);
+                        continue;
+                    },
+                };
+
+                // Try and parse value as yaml.
+                match serde_yaml::from_str(value) {
+                    Ok(value) => self.ipc_config.push((key.to_owned(), value)),
+                    Err(err) => error!("'{}': Invalid IPC config value: {:?}", option, err),
+                }
+            }
+        }
+
+        // Reload current config to pull new IPC config.
+        self.update_config(config);
+    }
+
     /// Process events for this terminal window.
     pub fn handle_event(
         &mut self,
         event_loop: &EventLoopWindowTarget<Event>,
         event_proxy: &EventLoopProxy<Event>,
-        config: &mut UiConfig,
         clipboard: &mut Clipboard,
         scheduler: &mut Scheduler,
         event: GlutinEvent<'_, Event>,
@@ -285,11 +338,11 @@ impl WindowContext {
             #[cfg(not(windows))]
             shell_pid: self.shell_pid,
             preserve_title: self.preserve_title,
+            config: &self.config,
             event_proxy,
             event_loop,
             clipboard,
             scheduler,
-            config,
         };
         let mut processor = input::Processor::new(context);
 
@@ -306,7 +359,7 @@ impl WindowContext {
                 &self.message_buffer,
                 &self.search_state,
                 old_is_searching,
-                config,
+                &self.config,
             );
             self.dirty = true;
         }
@@ -314,7 +367,7 @@ impl WindowContext {
         if self.dirty || self.mouse.hint_highlight_dirty {
             self.dirty |= self.display.update_highlighted_hints(
                 &terminal,
-                config,
+                &self.config,
                 &self.mouse,
                 self.modifiers,
             );
@@ -339,7 +392,7 @@ impl WindowContext {
             }
 
             // Redraw screen.
-            self.display.draw(terminal, &self.message_buffer, config, &self.search_state);
+            self.display.draw(terminal, &self.message_buffer, &self.config, &self.search_state);
         }
     }
 
diff --git a/alacritty_config/Cargo.toml b/alacritty_config/Cargo.toml
new file mode 100644
index 00000000..3b287f43
--- /dev/null
+++ b/alacritty_config/Cargo.toml
@@ -0,0 +1,14 @@
+[package]
+name = "alacritty_config"
+version = "0.1.0"
+authors = ["Christian Duerr <contact@christianduerr.com>"]
+license = "MIT/Apache-2.0"
+description = "Alacritty configuration abstractions"
+homepage = "https://github.com/alacritty/alacritty"
+edition = "2021"
+rust-version = "1.57.0"
+
+[dependencies]
+log = { version = "0.4.17", features = ["serde"] }
+serde_yaml = "0.8.24"
+serde = "1.0.137"
diff --git a/alacritty_config/src/lib.rs b/alacritty_config/src/lib.rs
new file mode 100644
index 00000000..7a467650
--- /dev/null
+++ b/alacritty_config/src/lib.rs
@@ -0,0 +1,64 @@
+use std::collections::HashMap;
+use std::error::Error;
+
+use log::LevelFilter;
+use serde::Deserialize;
+use serde_yaml::Value;
+
+pub trait SerdeReplace {
+    fn replace(&mut self, key: &str, value: Value) -> Result<(), Box<dyn Error>>;
+}
+
+macro_rules! impl_replace {
+    ($($ty:ty),*$(,)*) => {
+        $(
+            impl SerdeReplace for $ty {
+                fn replace(&mut self, key: &str, value: Value) -> Result<(), Box<dyn Error>> {
+                    replace_simple(self, key, value)
+                }
+            }
+        )*
+    };
+}
+
+#[rustfmt::skip]
+impl_replace!(
+    usize, u8, u16, u32, u64, u128,
+    isize, i8, i16, i32, i64, i128,
+    f32, f64,
+    bool,
+    char,
+    String,
+    LevelFilter,
+);
+
+fn replace_simple<'de, D>(data: &mut D, key: &str, value: Value) -> Result<(), Box<dyn Error>>
+where
+    D: Deserialize<'de>,
+{
+    if !key.is_empty() {
+        let error = format!("Fields \"{}\" do not exist", key);
+        return Err(error.into());
+    }
+    *data = D::deserialize(value)?;
+
+    Ok(())
+}
+
+impl<'de, T: Deserialize<'de>> SerdeReplace for Vec<T> {
+    fn replace(&mut self, key: &str, value: Value) -> Result<(), Box<dyn Error>> {
+        replace_simple(self, key, value)
+    }
+}
+
+impl<'de, T: Deserialize<'de>> SerdeReplace for Option<T> {
+    fn replace(&mut self, key: &str, value: Value) -> Result<(), Box<dyn Error>> {
+        replace_simple(self, key, value)
+    }
+}
+
+impl<'de, T: Deserialize<'de>> SerdeReplace for HashMap<String, T> {
+    fn replace(&mut self, key: &str, value: Value) -> Result<(), Box<dyn Error>> {
+        replace_simple(self, key, value)
+    }
+}
diff --git a/alacritty_config_derive/Cargo.toml b/alacritty_config_derive/Cargo.toml
index 8584d0d2..c901815e 100644
--- a/alacritty_config_derive/Cargo.toml
+++ b/alacritty_config_derive/Cargo.toml
@@ -16,7 +16,11 @@ syn = { version = "1.0.53", features = ["derive", "parsing", "proc-macro", "prin
 proc-macro2 = "1.0.24"
 quote = "1.0.7"
 
+[dev-dependencies.alacritty_config]
+path = "../alacritty_config"
+version = "0.1.0"
+
 [dev-dependencies]
+serde = { version = "1.0.117", features = ["derive"] }
 serde_yaml = "0.8.14"
-serde = "1.0.117"
 log = "0.4.11"
diff --git a/alacritty_config_derive/src/de_enum.rs b/alacritty_config_derive/src/config_deserialize/de_enum.rs
index 98247c0c..73634e73 100644
--- a/alacritty_config_derive/src/de_enum.rs
+++ b/alacritty_config_derive/src/config_deserialize/de_enum.rs
@@ -1,9 +1,11 @@
 use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
 use quote::{format_ident, quote};
-use syn::{DataEnum, Ident};
+use syn::{DataEnum, Generics, Ident};
 
-pub fn derive_deserialize(ident: Ident, data_enum: DataEnum) -> TokenStream {
+use crate::serde_replace;
+
+pub fn derive_deserialize(ident: Ident, generics: Generics, data_enum: DataEnum) -> TokenStream {
     let visitor = format_ident!("{}Visitor", ident);
 
     // Create match arm streams and get a list with all available values.
@@ -30,7 +32,7 @@ pub fn derive_deserialize(ident: Ident, data_enum: DataEnum) -> TokenStream {
     available_values.truncate(available_values.len().saturating_sub(2));
 
     // Generate deserialization impl.
-    let tokens = quote! {
+    let mut tokens = quote! {
         struct #visitor;
         impl<'de> serde::de::Visitor<'de> for #visitor {
             type Value = #ident;
@@ -62,5 +64,8 @@ pub fn derive_deserialize(ident: Ident, data_enum: DataEnum) -> TokenStream {
         }
     };
 
+    // Automatically implement [`alacritty_config::SerdeReplace`].
+    tokens.extend(serde_replace::derive_direct(ident, generics));
+
     tokens.into()
 }
diff --git a/alacritty_config_derive/src/de_struct.rs b/alacritty_config_derive/src/config_deserialize/de_struct.rs
index cf7ea141..4245764f 100644
--- a/alacritty_config_derive/src/de_struct.rs
+++ b/alacritty_config_derive/src/config_deserialize/de_struct.rs
@@ -1,13 +1,12 @@
 use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
 use quote::{format_ident, quote};
-use syn::parse::{self, Parse, ParseStream};
 use syn::punctuated::Punctuated;
 use syn::spanned::Spanned;
-use syn::{Error, Field, GenericParam, Generics, Ident, LitStr, Token, Type, TypeParam};
+use syn::{Error, Field, Generics, Ident, Type};
+
+use crate::{serde_replace, Attr, GenericsStreams, MULTIPLE_FLATTEN_ERROR};
 
-/// Error message when attempting to flatten multiple fields.
-const MULTIPLE_FLATTEN_ERROR: &str = "At most one instance of #[config(flatten)] is supported";
 /// Use this crate's name as log target.
 const LOG_TARGET: &str = env!("CARGO_PKG_NAME");
 
@@ -18,20 +17,20 @@ pub fn derive_deserialize<T>(
 ) -> TokenStream {
     // Create all necessary tokens for the implementation.
     let GenericsStreams { unconstrained, constrained, phantoms } =
-        generics_streams(generics.params);
+        crate::generics_streams(&generics.params);
     let FieldStreams { flatten, match_assignments } = fields_deserializer(&fields);
     let visitor = format_ident!("{}Visitor", ident);
 
     // Generate deserialization impl.
-    let tokens = quote! {
+    let mut tokens = quote! {
         #[derive(Default)]
         #[allow(non_snake_case)]
-        struct #visitor < #unconstrained > {
+        struct #visitor <#unconstrained> {
             #phantoms
         }
 
-        impl<'de, #constrained> serde::de::Visitor<'de> for #visitor < #unconstrained > {
-            type Value = #ident < #unconstrained >;
+        impl <'de, #constrained> serde::de::Visitor<'de> for #visitor <#unconstrained> {
+            type Value = #ident <#unconstrained>;
 
             fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
                 formatter.write_str("a mapping")
@@ -61,7 +60,7 @@ pub fn derive_deserialize<T>(
             }
         }
 
-        impl<'de, #constrained> serde::Deserialize<'de> for #ident < #unconstrained > {
+        impl <'de, #constrained> serde::Deserialize<'de> for #ident <#unconstrained> {
             fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
             where
                 D: serde::Deserializer<'de>,
@@ -71,6 +70,9 @@ pub fn derive_deserialize<T>(
         }
     };
 
+    // Automatically implement [`alacritty_config::SerdeReplace`].
+    tokens.extend(serde_replace::derive_recursive(ident, generics, fields));
+
     tokens.into()
 }
 
@@ -177,50 +179,3 @@ fn field_deserializer(field_streams: &mut FieldStreams, field: &Field) -> Result
 
     Ok(())
 }
-
-/// Field attribute.
-struct Attr {
-    ident: String,
-    param: Option<LitStr>,
-}
-
-impl Parse for Attr {
-    fn parse(input: ParseStream<'_>) -> parse::Result<Self> {
-        let ident = input.parse::<Ident>()?.to_string();
-        let param = input.parse::<Token![=]>().and_then(|_| input.parse()).ok();
-        Ok(Self { ident, param })
-    }
-}
-
-/// Storage for all necessary generics information.
-#[derive(Default)]
-struct GenericsStreams {
-    unconstrained: TokenStream2,
-    constrained: TokenStream2,
-    phantoms: TokenStream2,
-}
-
-/// Create the necessary generics annotations.
-///
-/// This will create three different token streams, which might look like this:
-///  - unconstrained: `T`
-///  - constrained: `T: Default + Deserialize<'de>`
-///  - phantoms: `T: PhantomData<T>,`
-fn generics_streams<T>(params: Punctuated<GenericParam, T>) -> GenericsStreams {
-    let mut generics = GenericsStreams::default();
-
-    for generic in params {
-        // NOTE: Lifetimes and const params are not supported.
-        if let GenericParam::Type(TypeParam { ident, .. }) = generic {
-            generics.unconstrained.extend(quote!( #ident , ));
-            generics.constrained.extend(quote! {
-                #ident : Default + serde::Deserialize<'de> ,
-            });
-            generics.phantoms.extend(quote! {
-                #ident : std::marker::PhantomData < #ident >,
-            });
-        }
-    }
-
-    generics
-}
diff --git a/alacritty_config_derive/src/config_deserialize/mod.rs b/alacritty_config_derive/src/config_deserialize/mod.rs
new file mode 100644
index 00000000..b1923377
--- /dev/null
+++ b/alacritty_config_derive/src/config_deserialize/mod.rs
@@ -0,0 +1,22 @@
+use proc_macro::TokenStream;
+use syn::{parse_macro_input, Data, DataStruct, DeriveInput, Error, Fields};
+
+/// Error if the derive was used on an unsupported type.
+const UNSUPPORTED_ERROR: &str = "ConfigDeserialize must be used on an enum or struct with fields";
+
+mod de_enum;
+mod de_struct;
+
+pub fn derive(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    match input.data {
+        Data::Struct(DataStruct { fields: Fields::Named(fields), .. }) => {
+            de_struct::derive_deserialize(input.ident, input.generics, fields.named)
+        },
+        Data::Enum(data_enum) => {
+            de_enum::derive_deserialize(input.ident, input.generics, data_enum)
+        },
+        _ => Error::new(input.ident.span(), UNSUPPORTED_ERROR).to_compile_error().into(),
+    }
+}
diff --git a/alacritty_config_derive/src/lib.rs b/alacritty_config_derive/src/lib.rs
index af8f2e7f..116d4828 100644
--- a/alacritty_config_derive/src/lib.rs
+++ b/alacritty_config_derive/src/lib.rs
@@ -2,25 +2,27 @@
 #![cfg_attr(feature = "cargo-clippy", deny(warnings))]
 
 use proc_macro::TokenStream;
-use syn::{parse_macro_input, Data, DataStruct, DeriveInput, Error, Fields, Path};
+use proc_macro2::TokenStream as TokenStream2;
+use quote::quote;
+use syn::parse::{self, Parse, ParseStream};
+use syn::punctuated::Punctuated;
+use syn::{GenericParam, Ident, LitStr, Path, Token, TypeParam};
 
-mod de_enum;
-mod de_struct;
+mod config_deserialize;
+mod serde_replace;
 
-/// Error if the derive was used on an unsupported type.
-const UNSUPPORTED_ERROR: &str = "ConfigDeserialize must be used on a struct with fields";
+/// Error message when attempting to flatten multiple fields.
+pub(crate) const MULTIPLE_FLATTEN_ERROR: &str =
+    "At most one instance of #[config(flatten)] is supported";
 
 #[proc_macro_derive(ConfigDeserialize, attributes(config))]
 pub fn derive_config_deserialize(input: TokenStream) -> TokenStream {
-    let input = parse_macro_input!(input as DeriveInput);
-
-    match input.data {
-        Data::Struct(DataStruct { fields: Fields::Named(fields), .. }) => {
-            de_struct::derive_deserialize(input.ident, input.generics, fields.named)
-        },
-        Data::Enum(data_enum) => de_enum::derive_deserialize(input.ident, data_enum),
-        _ => Error::new(input.ident.span(), UNSUPPORTED_ERROR).to_compile_error().into(),
-    }
+    config_deserialize::derive(input)
+}
+
+#[proc_macro_derive(SerdeReplace)]
+pub fn derive_serde_replace(input: TokenStream) -> TokenStream {
+    serde_replace::derive(input)
 }
 
 /// Verify that a token path ends with a specific segment.
@@ -28,3 +30,50 @@ pub(crate) fn path_ends_with(path: &Path, segment: &str) -> bool {
     let segments = path.segments.iter();
     segments.last().map_or(false, |s| s.ident == segment)
 }
+
+/// Storage for all necessary generics information.
+#[derive(Default)]
+struct GenericsStreams {
+    unconstrained: TokenStream2,
+    constrained: TokenStream2,
+    phantoms: TokenStream2,
+}
+
+/// Create the necessary generics annotations.
+///
+/// This will create three different token streams, which might look like this:
+///  - unconstrained: `T`
+///  - constrained: `T: Default + Deserialize<'de>`
+///  - phantoms: `T: PhantomData<T>,`
+pub(crate) fn generics_streams<T>(params: &Punctuated<GenericParam, T>) -> GenericsStreams {
+    let mut generics = GenericsStreams::default();
+
+    for generic in params {
+        // NOTE: Lifetimes and const params are not supported.
+        if let GenericParam::Type(TypeParam { ident, .. }) = generic {
+            generics.unconstrained.extend(quote!( #ident , ));
+            generics.constrained.extend(quote! {
+                #ident : Default + serde::Deserialize<'de> + alacritty_config::SerdeReplace,
+            });
+            generics.phantoms.extend(quote! {
+                #ident : std::marker::PhantomData < #ident >,
+            });
+        }
+    }
+
+    generics
+}
+
+/// Field attribute.
+pub(crate) struct Attr {
+    ident: String,
+    param: Option<LitStr>,
+}
+
+impl Parse for Attr {
+    fn parse(input: ParseStream<'_>) -> parse::Result<Self> {
+        let ident = input.parse::<Ident>()?.to_string();
+        let param = input.parse::<Token![=]>().and_then(|_| input.parse()).ok();
+        Ok(Self { ident, param })
+    }
+}
diff --git a/alacritty_config_derive/src/serde_replace.rs b/alacritty_config_derive/src/serde_replace.rs
new file mode 100644
index 00000000..4a0a6a99
--- /dev/null
+++ b/alacritty_config_derive/src/serde_replace.rs
@@ -0,0 +1,113 @@
+use proc_macro::TokenStream;
+use proc_macro2::TokenStream as TokenStream2;
+use quote::quote;
+use syn::punctuated::Punctuated;
+use syn::{
+    parse_macro_input, Data, DataStruct, DeriveInput, Error, Field, Fields, Generics, Ident,
+};
+
+use crate::{Attr, GenericsStreams, MULTIPLE_FLATTEN_ERROR};
+
+/// Error if the derive was used on an unsupported type.
+const UNSUPPORTED_ERROR: &str = "SerdeReplace must be used on a tuple struct";
+
+pub fn derive(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    match input.data {
+        Data::Struct(DataStruct { fields: Fields::Unnamed(_), .. }) | Data::Enum(_) => {
+            derive_direct(input.ident, input.generics).into()
+        },
+        Data::Struct(DataStruct { fields: Fields::Named(fields), .. }) => {
+            derive_recursive(input.ident, input.generics, fields.named).into()
+        },
+        _ => Error::new(input.ident.span(), UNSUPPORTED_ERROR).to_compile_error().into(),
+    }
+}
+
+pub fn derive_direct(ident: Ident, generics: Generics) -> TokenStream2 {
+    quote! {
+        impl <#generics> alacritty_config::SerdeReplace for #ident <#generics> {
+            fn replace(&mut self, key: &str, value: serde_yaml::Value) -> Result<(), Box<dyn std::error::Error>> {
+                if !key.is_empty() {
+                    let error = format!("Fields \"{}\" do not exist", key);
+                    return Err(error.into());
+                }
+                *self = serde::Deserialize::deserialize(value)?;
+
+                Ok(())
+            }
+        }
+    }
+}
+
+pub fn derive_recursive<T>(
+    ident: Ident,
+    generics: Generics,
+    fields: Punctuated<Field, T>,
+) -> TokenStream2 {
+    let GenericsStreams { unconstrained, constrained, .. } =
+        crate::generics_streams(&generics.params);
+    let replace_arms = match_arms(&fields);
+
+    quote! {
+        #[allow(clippy::extra_unused_lifetimes)]
+        impl <'de, #constrained> alacritty_config::SerdeReplace for #ident <#unconstrained> {
+            fn replace(&mut self, key: &str, value: serde_yaml::Value) -> Result<(), Box<dyn std::error::Error>> {
+                if key.is_empty() {
+                    *self = serde::Deserialize::deserialize(value)?;
+                    return Ok(());
+                }
+
+                let (field, next_key) = key.split_once('.').unwrap_or((key, ""));
+                match field {
+                    #replace_arms
+                    _ => {
+                        let error = format!("Field \"{}\" does not exist", field);
+                        return Err(error.into());
+                    },
+                }
+
+                Ok(())
+            }
+        }
+    }
+}
+
+/// Create SerdeReplace recursive match arms.
+fn match_arms<T>(fields: &Punctuated<Field, T>) -> TokenStream2 {
+    let mut stream = TokenStream2::default();
+    let mut flattened_arm = None;
+
+    // Create arm for each field.
+    for field in fields {
+        let ident = field.ident.as_ref().expect("unreachable tuple struct");
+        let literal = ident.to_string();
+
+        // Check if #[config(flattened)] attribute is present.
+        let flatten = field
+            .attrs
+            .iter()
+            .filter_map(|attr| attr.parse_args::<Attr>().ok())
+            .any(|parsed| parsed.ident.as_str() == "flatten");
+
+        if flatten && flattened_arm.is_some() {
+            return Error::new(ident.span(), MULTIPLE_FLATTEN_ERROR).to_compile_error();
+        } else if flatten {
+            flattened_arm = Some(quote! {
+                _ => alacritty_config::SerdeReplace::replace(&mut self.#ident, key, value)?,
+            });
+        } else {
+            stream.extend(quote! {
+                #literal => alacritty_config::SerdeReplace::replace(&mut self.#ident, next_key, value)?,
+            });
+        }
+    }
+
+    // Add the flattened catch-all as last match arm.
+    if let Some(flattened_arm) = flattened_arm.take() {
+        stream.extend(flattened_arm);
+    }
+
+    stream
+}
diff --git a/alacritty_config_derive/tests/config.rs b/alacritty_config_derive/tests/config.rs
index 4828b822..bd449ff8 100644
--- a/alacritty_config_derive/tests/config.rs
+++ b/alacritty_config_derive/tests/config.rs
@@ -1,8 +1,10 @@
 use std::sync::{Arc, Mutex};
 
 use log::{Level, Log, Metadata, Record};
+use serde::Deserialize;
 
-use alacritty_config_derive::ConfigDeserialize;
+use alacritty_config::SerdeReplace as _;
+use alacritty_config_derive::{ConfigDeserialize, SerdeReplace};
 
 #[derive(ConfigDeserialize, Debug, PartialEq, Eq)]
 enum TestEnum {
@@ -63,6 +65,7 @@ struct Test2<T: Default> {
     field3: usize,
     #[config(alias = "aliased")]
     field4: u8,
+    newtype: NewType,
 }
 
 #[derive(ConfigDeserialize, Default)]
@@ -70,6 +73,9 @@ struct Test3 {
     flatty: usize,
 }
 
+#[derive(SerdeReplace, Deserialize, Default, PartialEq, Eq, Debug)]
+struct NewType(usize);
+
 #[test]
 fn config_deserialize() {
     let logger = unsafe {
@@ -159,3 +165,33 @@ impl Log for Logger {
 
     fn flush(&self) {}
 }
+
+#[test]
+fn field_replacement() {
+    let mut test = Test::default();
+
+    let value = serde_yaml::to_value(13).unwrap();
+    test.replace("nesting.field2", value).unwrap();
+
+    assert_eq!(test.nesting.field2, Some(13));
+}
+
+#[test]
+fn replace_derive() {
+    let mut test = Test::default();
+
+    let value = serde_yaml::to_value(9).unwrap();
+    test.replace("nesting.newtype", value).unwrap();
+
+    assert_eq!(test.nesting.newtype, NewType(9));
+}
+
+#[test]
+fn replace_flatten() {
+    let mut test = Test::default();
+
+    let value = serde_yaml::to_value(7).unwrap();
+    test.replace("flatty", value).unwrap();
+
+    assert_eq!(test.flatten.flatty, 7);
+}
diff --git a/alacritty_terminal/Cargo.toml b/alacritty_terminal/Cargo.toml
index fb5d2d22..06d3961a 100644
--- a/alacritty_terminal/Cargo.toml
+++ b/alacritty_terminal/Cargo.toml
@@ -13,6 +13,10 @@ rust-version = "1.57.0"
 path = "../alacritty_config_derive"
 version = "0.1.0"
 
+[dependencies.alacritty_config]
+path = "../alacritty_config"
+version = "0.1.0"
+
 [dependencies]
 libc = "0.2"
 bitflags = "1"
diff --git a/alacritty_terminal/src/config/mod.rs b/alacritty_terminal/src/config/mod.rs
index 5822d591..53a0eb77 100644
--- a/alacritty_terminal/src/config/mod.rs
+++ b/alacritty_terminal/src/config/mod.rs
@@ -4,7 +4,7 @@ use std::path::PathBuf;
 
 use serde::Deserialize;
 
-use alacritty_config_derive::ConfigDeserialize;
+use alacritty_config_derive::{ConfigDeserialize, SerdeReplace};
 
 mod scrolling;
 
@@ -17,7 +17,7 @@ pub const LOG_TARGET_CONFIG: &str = "alacritty_config_derive";
 const MIN_BLINK_INTERVAL: u64 = 10;
 
 /// Top-level config type.
-#[derive(ConfigDeserialize, Debug, PartialEq, Default)]
+#[derive(ConfigDeserialize, Clone, Debug, PartialEq, Default)]
 pub struct Config {
     /// TERM env variable.
     pub env: HashMap<String, String>,
@@ -125,7 +125,7 @@ impl Cursor {
     }
 }
 
-#[derive(Deserialize, Debug, Copy, Clone, PartialEq, Eq)]
+#[derive(SerdeReplace, Deserialize, Debug, Copy, Clone, PartialEq, Eq)]
 #[serde(untagged)]
 pub enum ConfigCursorStyle {
     Shape(CursorShape),
@@ -222,7 +222,7 @@ impl Program {
 }
 
 /// Wrapper around f32 that represents a percentage value between 0.0 and 1.0.
-#[derive(Deserialize, Clone, Copy, Debug, PartialEq)]
+#[derive(SerdeReplace, Deserialize, Clone, Copy, Debug, PartialEq)]
 pub struct Percentage(f32);
 
 impl Default for Percentage {
diff --git a/alacritty_terminal/src/config/scrolling.rs b/alacritty_terminal/src/config/scrolling.rs
index 9a5a718c..f4e55787 100644
--- a/alacritty_terminal/src/config/scrolling.rs
+++ b/alacritty_terminal/src/config/scrolling.rs
@@ -1,7 +1,7 @@
 use serde::de::Error as SerdeError;
 use serde::{Deserialize, Deserializer};
 
-use alacritty_config_derive::ConfigDeserialize;
+use alacritty_config_derive::{ConfigDeserialize, SerdeReplace};
 
 /// Maximum scrollback amount configurable.
 pub const MAX_SCROLLBACK_LINES: u32 = 100_000;
@@ -31,7 +31,7 @@ impl Scrolling {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(SerdeReplace, Copy, Clone, Debug, PartialEq, Eq)]
 struct ScrollingHistory(u32);
 
 impl Default for ScrollingHistory {
diff --git a/alacritty_terminal/src/index.rs b/alacritty_terminal/src/index.rs
index e672c752..9464b8d8 100644
--- a/alacritty_terminal/src/index.rs
+++ b/alacritty_terminal/src/index.rs
@@ -7,6 +7,8 @@ use std::ops::{Add, AddAssign, Deref, Sub, SubAssign};
 
 use serde::{Deserialize, Serialize};
 
+use alacritty_config_derive::SerdeReplace;
+
 use crate::grid::Dimensions;
 
 /// The side of a cell.
@@ -222,7 +224,19 @@ impl PartialEq<usize> for Line {
 /// A column.
 ///
 /// Newtype to avoid passing values incorrectly.
-#[derive(Serialize, Deserialize, Debug, Copy, Clone, Eq, PartialEq, Default, Ord, PartialOrd)]
+#[derive(
+    SerdeReplace,
+    Serialize,
+    Deserialize,
+    Debug,
+    Copy,
+    Clone,
+    Eq,
+    PartialEq,
+    Default,
+    Ord,
+    PartialOrd,
+)]
 pub struct Column(pub usize);
 
 impl fmt::Display for Column {
diff --git a/alacritty_terminal/src/term/color.rs b/alacritty_terminal/src/term/color.rs
index 1cfdec6b..3c545b28 100644
--- a/alacritty_terminal/src/term/color.rs
+++ b/alacritty_terminal/src/term/color.rs
@@ -7,12 +7,14 @@ use serde::de::{Error as _, Visitor};
 use serde::{Deserialize, Deserializer, Serialize};
 use serde_yaml::Value;
 
+use alacritty_config_derive::SerdeReplace;
+
 use crate::ansi::NamedColor;
 
 /// Number of terminal colors.
 pub const COUNT: usize = 269;
 
-#[derive(Debug, Eq, PartialEq, Copy, Clone, Default, Serialize)]
+#[derive(SerdeReplace, Debug, Eq, PartialEq, Copy, Clone, Default, Serialize)]
 pub struct Rgb {
     pub r: u8,
     pub g: u8,
@@ -170,7 +172,7 @@ impl FromStr for Rgb {
 }
 
 /// RGB color optionally referencing the cell's foreground or background.
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(SerdeReplace, Copy, Clone, Debug, PartialEq, Eq)]
 pub enum CellRgb {
     CellForeground,
     CellBackground,
diff --git a/alacritty_terminal/src/tty/unix.rs b/alacritty_terminal/src/tty/unix.rs
index f52f0920..63da4f9c 100644
--- a/alacritty_terminal/src/tty/unix.rs
+++ b/alacritty_terminal/src/tty/unix.rs
@@ -148,7 +148,7 @@ fn default_shell_command(pw: &Passwd<'_>) -> Command {
 }
 
 /// Create a new TTY and return a handle to interact with it.
-pub fn new(config: &PtyConfig, window_size: WindowSize, window_id: Option<usize>) -> Result<Pty> {
+pub fn new(config: &PtyConfig, window_size: WindowSize, window_id: u64) -> Result<Pty> {
     let (master, slave) = make_pty(window_size.to_winsize())?;
 
     #[cfg(any(target_os = "linux", target_os = "macos"))]
@@ -178,13 +178,14 @@ pub fn new(config: &PtyConfig, window_size: WindowSize, window_id: Option<usize>
     builder.stdout(unsafe { Stdio::from_raw_fd(slave) });
 
     // Setup shell environment.
+    let window_id = window_id.to_string();
+    builder.env("ALACRITTY_WINDOW_ID", &window_id);
     builder.env("LOGNAME", pw.name);
     builder.env("USER", pw.name);
     builder.env("HOME", pw.dir);
 
-    if let Some(window_id) = window_id {
-        builder.env("WINDOWID", format!("{}", window_id));
-    }
+    // Set Window ID for clients relying on X11 hacks.
+    builder.env("WINDOWID", window_id);
 
     unsafe {
         builder.pre_exec(move || {
diff --git a/alacritty_terminal/src/tty/windows/mod.rs b/alacritty_terminal/src/tty/windows/mod.rs
index aa21ce14..57925f4c 100644
--- a/alacritty_terminal/src/tty/windows/mod.rs
+++ b/alacritty_terminal/src/tty/windows/mod.rs
@@ -27,7 +27,7 @@ pub struct Pty {
     child_watcher: ChildExitWatcher,
 }
 
-pub fn new(config: &PtyConfig, window_size: WindowSize, _window_id: Option<usize>) -> Result<Pty> {
+pub fn new(config: &PtyConfig, window_size: WindowSize, _window_id: u64) -> Result<Pty> {
     conpty::new(config, window_size)
         .ok_or_else(|| Error::new(ErrorKind::Other, "failed to spawn conpty"))
 }
diff --git a/extra/alacritty-msg.man b/extra/alacritty-msg.man
index ed600d40..f4a34f46 100644
--- a/extra/alacritty-msg.man
+++ b/extra/alacritty-msg.man
@@ -10,20 +10,43 @@ making it possible to control Alacritty without directly accessing it.
 \fB\-s\fR, \fB\-\-socket\fR <socket>
 Path for IPC socket creation
 .SH "MESSAGES"
+.TP
 \fBcreate-window\fR
 Create a new window in the same Alacritty process
 .TP
 .SH "\tOPTIONS"
 .RS 12
+.TP
 \fB\-\-hold\fR
 Remain open after child process exits
-
+.TP
 \fB\-\-working\-directory\fR <working\-directory>
 Start the shell in the specified working directory
-
+.TP
 \fB\-e\fR, \fB\-\-command\fR <command>...
 Command and args to execute (must be last argument)
 .RE
+.TP
+\fBconfig\fR
+Update the Alacritty configuration
+.TP
+.SH "\tARGS"
+.RS 12
+.TP
+\fB<CONFIG_OPTIONS>...\fR
+Configuration file options [example: cursor.style=Beam]
+.RE
+.TP
+.SH "\tOPTIONS"
+.RS 12
+.TP
+\fB\-w\fR, \fB\-\-window\-id\fR <WINDOW_ID>
+Window ID for the new config.
+
+Use `-1` to apply this change to all windows.
+
+[default: \fB$ALACRITTY_WINDOW_ID\fR]
+.RE
 .SH "SEE ALSO"
 See the alacritty github repository at https://github.com/alacritty/alacritty for the full documentation.
 .SH "BUGS"
diff --git a/extra/completions/_alacritty b/extra/completions/_alacritty
index 50a5c00d..6b80a797 100644
--- a/extra/completions/_alacritty
+++ b/extra/completions/_alacritty
@@ -73,6 +73,17 @@ _arguments "${_arguments_options[@]}" \
 '--help[Print help information]' \
 && ret=0
 ;;
+(config)
+_arguments "${_arguments_options[@]}" \
+'-w+[Window ID for the new config]:WINDOW_ID: ' \
+'--window-id=[Window ID for the new config]:WINDOW_ID: ' \
+'()-r[Clear all runtime configuration changes]' \
+'()--reset[Clear all runtime configuration changes]' \
+'-h[Print help information]' \
+'--help[Print help information]' \
+'*::options -- Configuration file options \[example\: cursor.style=Beam\]:' \
+&& ret=0
+;;
 (help)
 _arguments "${_arguments_options[@]}" \
 '*::subcommand -- The subcommand whose help message to display:' \
@@ -100,6 +111,11 @@ _alacritty_commands() {
     )
     _describe -t commands 'alacritty commands' commands "$@"
 }
+(( $+functions[_alacritty__msg__config_commands] )) ||
+_alacritty__msg__config_commands() {
+    local commands; commands=()
+    _describe -t commands 'alacritty msg config commands' commands "$@"
+}
 (( $+functions[_alacritty__msg__create-window_commands] )) ||
 _alacritty__msg__create-window_commands() {
     local commands; commands=()
@@ -119,6 +135,7 @@ _alacritty__msg__help_commands() {
 _alacritty__msg_commands() {
     local commands; commands=(
 'create-window:Create a new window in the same Alacritty process' \
+'config:Update the Alacritty configuration' \
 'help:Print this message or the help of the given subcommand(s)' \
     )
     _describe -t commands 'alacritty msg commands' commands "$@"
diff --git a/extra/completions/alacritty.bash b/extra/completions/alacritty.bash
index 428e9583..5cca6466 100644
--- a/extra/completions/alacritty.bash
+++ b/extra/completions/alacritty.bash
@@ -12,6 +12,9 @@ _alacritty() {
             "$1")
                 cmd="alacritty"
                 ;;
+            config)
+                cmd+="__config"
+                ;;
             create-window)
                 cmd+="__create__window"
                 ;;
@@ -100,7 +103,7 @@ _alacritty() {
             return 0
             ;;
         alacritty__msg)
-            opts="-s -h --socket --help create-window help"
+            opts="-s -h --socket --help create-window config help"
             if [[ ${cur} == -* || ${COMP_CWORD} -eq 2 ]] ; then
                 COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
                 return 0
@@ -121,6 +124,28 @@ _alacritty() {
             COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
             return 0
             ;;
+        alacritty__msg__config)
+            opts="-w -r -h --window-id --reset --help <CONFIG_OPTIONS>..."
+            if [[ ${cur} == -* || ${COMP_CWORD} -eq 3 ]] ; then
+                COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
+                return 0
+            fi
+            case "${prev}" in
+                --window-id)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                -w)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                *)
+                    COMPREPLY=()
+                    ;;
+            esac
+            COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
+            return 0
+            ;;
         alacritty__msg__create__window)
             opts="-e -t -h --working-directory --hold --command --title --class --help"
             if [[ ${cur} == -* || ${COMP_CWORD} -eq 3 ]] ; then
diff --git a/extra/completions/alacritty.fish b/extra/completions/alacritty.fish
index 69b3d777..fdc24ab2 100644
--- a/extra/completions/alacritty.fish
+++ b/extra/completions/alacritty.fish
@@ -15,13 +15,17 @@ complete -c alacritty -n "__fish_use_subcommand" -s v -d 'Increases the level of
 complete -c alacritty -n "__fish_use_subcommand" -l hold -d 'Remain open after child process exit'
 complete -c alacritty -n "__fish_use_subcommand" -f -a "msg" -d 'Send a message to the Alacritty socket'
 complete -c alacritty -n "__fish_use_subcommand" -f -a "help" -d 'Print this message or the help of the given subcommand(s)'
-complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from help" -s s -l socket -d 'IPC socket connection path override' -r -F
-complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from help" -s h -l help -d 'Print help information'
-complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from help" -f -a "create-window" -d 'Create a new window in the same Alacritty process'
-complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from help" -f -a "help" -d 'Print this message or the help of the given subcommand(s)'
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from config; and not __fish_seen_subcommand_from help" -s s -l socket -d 'IPC socket connection path override' -r -F
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from config; and not __fish_seen_subcommand_from help" -s h -l help -d 'Print help information'
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from config; and not __fish_seen_subcommand_from help" -f -a "create-window" -d 'Create a new window in the same Alacritty process'
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from config; and not __fish_seen_subcommand_from help" -f -a "config" -d 'Update the Alacritty configuration'
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and not __fish_seen_subcommand_from create-window; and not __fish_seen_subcommand_from config; and not __fish_seen_subcommand_from help" -f -a "help" -d 'Print this message or the help of the given subcommand(s)'
 complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from create-window" -l working-directory -d 'Start the shell in the specified working directory' -r -F
 complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from create-window" -s e -l command -d 'Command and args to execute (must be last argument)' -r
 complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from create-window" -s t -l title -d 'Defines the window title [default: Alacritty]' -r
 complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from create-window" -l class -d 'Defines window class/app_id on X11/Wayland [default: Alacritty]' -r
 complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from create-window" -l hold -d 'Remain open after child process exit'
 complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from create-window" -s h -l help -d 'Print help information'
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from config" -s w -l window-id -d 'Window ID for the new config' -r
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from config" -s r -l reset -d 'Clear all runtime configuration changes'
+complete -c alacritty -n "__fish_seen_subcommand_from msg; and __fish_seen_subcommand_from config" -s h -l help -d 'Print help information'