Add support for title stack escape sequences

This commit adds the concept of a "title stack" to the terminal. Some programs
(e.g. vim) send control sequences `CSI 22 ; 0` (push title) and `CSI 23 ; 0`
(pop title).

The title stack is just a history of previous titles.  Applications can push
the current title onto the stack, and pop it back off (setting the window title
in the process).

Fixes #2840.

7 files changed, 136 insertions, 34 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8bc0097d..ce817613 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `ReceiveChar` key binding action to insert the key's text character
 - Live reload font size from config
 - New CLI flag `--hold` for keeping Alacritty opened after its child process exits
+- Escape sequence to save and restore window title from stack
 
 ### Changed
 
diff --git a/alacritty/src/cli.rs b/alacritty/src/cli.rs
index 99387a61..b0e3cc94 100644
--- a/alacritty/src/cli.rs
+++ b/alacritty/src/cli.rs
@@ -266,9 +266,12 @@ impl Options {
 
         config.window.dimensions = self.dimensions.unwrap_or(config.window.dimensions);
         config.window.position = self.position.or(config.window.position);
-        config.window.title = self.title.or(config.window.title);
         config.window.embed = self.embed.and_then(|embed| embed.parse().ok());
 
+        if let Some(title) = self.title {
+            config.window.title = title.clone();
+        }
+
         if let Some(class) = self.class {
             let parts: Vec<_> = class.split(',').collect();
             config.window.class.instance = parts[0].into();
@@ -277,7 +280,7 @@ impl Options {
             }
         }
 
-        config.set_dynamic_title(config.dynamic_title() && config.window.title.is_none());
+        config.set_dynamic_title(config.dynamic_title() && config.window.title == DEFAULT_NAME);
 
         config.debug.print_events = self.print_events || config.debug.print_events;
         config.debug.log_level = max(config.debug.log_level, self.log_level);
@@ -298,7 +301,8 @@ mod test {
 
     #[test]
     fn dynamic_title_ignoring_options_by_default() {
-        let config = Config::default();
+        let mut config = Config::default();
+        config.window.title = "Alacritty".to_string();
         let old_dynamic_title = config.dynamic_title();
 
         let config = Options::default().into_config(config);
@@ -321,7 +325,7 @@ mod test {
     fn dynamic_title_overridden_by_config() {
         let mut config = Config::default();
 
-        config.window.title = Some("foo".to_owned());
+        config.window.title = "foo".to_owned();
         let config = Options::default().into_config(config);
 
         assert!(!config.dynamic_title());
diff --git a/alacritty/src/window.rs b/alacritty/src/window.rs
index c783dc69..0394296e 100644
--- a/alacritty/src/window.rs
+++ b/alacritty/src/window.rs
@@ -31,7 +31,7 @@ use image::ImageFormat;
 #[cfg(not(any(target_os = "macos", windows)))]
 use x11_dl::xlib::{Display as XDisplay, PropModeReplace, XErrorEvent, Xlib};
 
-use alacritty_terminal::config::{Decorations, StartupMode, WindowConfig, DEFAULT_NAME};
+use alacritty_terminal::config::{Decorations, StartupMode, WindowConfig};
 use alacritty_terminal::event::Event;
 use alacritty_terminal::gl;
 use alacritty_terminal::term::{SizeInfo, Term};
@@ -146,9 +146,7 @@ impl Window {
         config: &Config,
         logical: Option<LogicalSize>,
     ) -> Result<Window> {
-        let title = config.window.title.as_ref().map_or(DEFAULT_NAME, |t| t);
-
-        let window_builder = Window::get_platform_window(title, &config.window);
+        let window_builder = Window::get_platform_window(&config.window.title, &config.window);
         let windowed_context =
             create_gl_window(window_builder.clone(), &event_loop, false, logical)
                 .or_else(|_| create_gl_window(window_builder, &event_loop, true, logical))?;
diff --git a/alacritty_terminal/src/ansi.rs b/alacritty_terminal/src/ansi.rs
index b34c1cd9..493b02aa 100644
--- a/alacritty_terminal/src/ansi.rs
+++ b/alacritty_terminal/src/ansi.rs
@@ -328,6 +328,12 @@ pub trait Handler {
 
     /// Run the dectest routine
     fn dectest(&mut self) {}
+
+    /// Push a title onto the stack
+    fn push_title(&mut self) {}
+
+    /// Pop the last title from the stack
+    fn pop_title(&mut self) {}
 }
 
 /// Describes shape of cursor
@@ -412,7 +418,13 @@ impl Mode {
     /// Create mode from a primitive
     ///
     /// TODO lots of unhandled values..
-    pub fn from_primitive(private: bool, num: i64) -> Option<Mode> {
+    pub fn from_primitive(intermediate: Option<&u8>, num: i64) -> Option<Mode> {
+        let private = match intermediate {
+            Some(b'?') => true,
+            None => false,
+            _ => return None,
+        };
+
         if private {
             Some(match num {
                 1 => Mode::CursorKeys,
@@ -991,22 +1003,18 @@ where
                 handler.clear_line(mode);
             },
             ('S', None) => handler.scroll_up(Line(arg_or_default!(idx: 0, default: 1) as usize)),
+            ('t', None) => match arg_or_default!(idx: 0, default: 1) as usize {
+                22 => handler.push_title(),
+                23 => handler.pop_title(),
+                _ => unhandled!(),
+            },
             ('T', None) => handler.scroll_down(Line(arg_or_default!(idx: 0, default: 1) as usize)),
             ('L', None) => {
                 handler.insert_blank_lines(Line(arg_or_default!(idx: 0, default: 1) as usize))
             },
             ('l', intermediate) => {
-                let is_private_mode = match intermediate {
-                    Some(b'?') => true,
-                    None => false,
-                    _ => {
-                        unhandled!();
-                        return;
-                    },
-                };
                 for arg in args {
-                    let mode = Mode::from_primitive(is_private_mode, *arg);
-                    match mode {
+                    match Mode::from_primitive(intermediate, *arg) {
                         Some(mode) => handler.unset_mode(mode),
                         None => {
                             unhandled!();
@@ -1027,17 +1035,8 @@ where
                 handler.goto_line(Line(arg_or_default!(idx: 0, default: 1) as usize - 1))
             },
             ('h', intermediate) => {
-                let is_private_mode = match intermediate {
-                    Some(b'?') => true,
-                    None => false,
-                    _ => {
-                        unhandled!();
-                        return;
-                    },
-                };
                 for arg in args {
-                    let mode = Mode::from_primitive(is_private_mode, *arg);
-                    match mode {
+                    match Mode::from_primitive(intermediate, *arg) {
                         Some(mode) => handler.set_mode(mode),
                         None => {
                             unhandled!();
diff --git a/alacritty_terminal/src/config/window.rs b/alacritty_terminal/src/config/window.rs
index f470f936..49bd70bb 100644
--- a/alacritty_terminal/src/config/window.rs
+++ b/alacritty_terminal/src/config/window.rs
@@ -36,8 +36,8 @@ pub struct WindowConfig {
     startup_mode: StartupMode,
 
     /// Window title
-    #[serde(deserialize_with = "failure_default")]
-    pub title: Option<String>,
+    #[serde(default = "default_title")]
+    pub title: String,
 
     /// Window class
     #[serde(deserialize_with = "from_string_or_deserialize")]
@@ -56,6 +56,10 @@ pub struct WindowConfig {
     pub start_maximized: Option<bool>,
 }
 
+pub fn default_title() -> String {
+    DEFAULT_NAME.to_string()
+}
+
 impl WindowConfig {
     pub fn startup_mode(&self) -> StartupMode {
         match self.start_maximized {
diff --git a/alacritty_terminal/src/term/mod.rs b/alacritty_terminal/src/term/mod.rs
index 4e51d734..382b10a9 100644
--- a/alacritty_terminal/src/term/mod.rs
+++ b/alacritty_terminal/src/term/mod.rs
@@ -27,7 +27,7 @@ use crate::ansi::{
     self, Attr, CharsetIndex, Color, CursorStyle, Handler, NamedColor, StandardCharset, TermInfo,
 };
 use crate::clipboard::{Clipboard, ClipboardType};
-use crate::config::{Config, VisualBellAnimation};
+use crate::config::{Config, VisualBellAnimation, DEFAULT_NAME};
 use crate::cursor::CursorKey;
 use crate::event::{Event, EventListener};
 use crate::grid::{
@@ -47,6 +47,9 @@ pub mod color;
 /// Used to match equal brackets, when performing a bracket-pair selection.
 const BRACKET_PAIRS: [(char, char); 4] = [('(', ')'), ('[', ']'), ('{', '}'), ('<', '>')];
 
+/// Max size of the window title stack
+const TITLE_STACK_MAX_DEPTH: usize = 4096;
+
 /// A type that can expand a given point to a region
 ///
 /// Usually this is implemented for some 2-D array type since
@@ -760,6 +763,13 @@ pub struct Term<T> {
 
     /// Terminal focus
     pub is_focused: bool,
+
+    /// Current title of the window
+    title: String,
+
+    /// Stack of saved window titles. When a title is popped from this stack, the `title` for the
+    /// term is set, and the Glutin window's title attribute is changed through the event listener.
+    title_stack: Vec<String>,
 }
 
 /// Terminal size info
@@ -887,6 +897,8 @@ impl<T> Term<T> {
             clipboard,
             event_proxy,
             is_focused: true,
+            title: config.window.title.clone(),
+            title_stack: Vec::new(),
         }
     }
 
@@ -1322,6 +1334,9 @@ impl<T: EventListener> ansi::Handler for Term<T> {
     #[cfg(not(windows))]
     fn set_title(&mut self, title: &str) {
         if self.dynamic_title {
+            trace!("Setting window title to '{}'", title);
+
+            self.title = title.into();
             self.event_proxy.send_event(Event::Title(title.to_owned()));
         }
     }
@@ -1336,13 +1351,16 @@ impl<T: EventListener> ansi::Handler for Term<T> {
             //
             // The starts_with check is necessary because other shells e.g. bash set a
             // different title and don't need Alacritty prepended.
+            trace!("Setting window title to '{}'", title);
+
             let title = if !tty::is_conpty() && title.starts_with(' ') {
                 format!("Alacritty {}", title.trim())
             } else {
                 title.to_owned()
             };
 
-            self.event_proxy.send_event(Event::Title(title));
+            self.title = title.clone();
+            self.event_proxy.send_event(Event::Title(title.to_owned()));
         }
     }
 
@@ -1912,6 +1930,8 @@ impl<T: EventListener> ansi::Handler for Term<T> {
         self.grid.reset(&Cell::default());
         self.alt_grid.reset(&Cell::default());
         self.scroll_region = Line(0)..self.grid.num_lines();
+        self.title = DEFAULT_NAME.to_string();
+        self.title_stack.clear();
     }
 
     #[inline]
@@ -2089,6 +2109,31 @@ impl<T: EventListener> ansi::Handler for Term<T> {
         trace!("Setting cursor style {:?}", style);
         self.cursor_style = style;
     }
+
+    #[inline]
+    fn push_title(&mut self) {
+        trace!("Pushing '{}' onto title stack", self.title);
+
+        if self.title_stack.len() >= TITLE_STACK_MAX_DEPTH {
+            let removed = self.title_stack.remove(0);
+            trace!(
+                "Removing '{}' from bottom of title stack that exceeds its maximum depth",
+                removed
+            );
+        }
+
+        self.title_stack.push(self.title.clone());
+    }
+
+    #[inline]
+    fn pop_title(&mut self) {
+        trace!("Attempting to pop title from stack...");
+
+        if let Some(popped) = self.title_stack.pop() {
+            trace!("Title '{}' popped from stack", popped);
+            self.set_title(&popped);
+        }
+    }
 }
 
 struct TabStops {
@@ -2301,6 +2346,57 @@ mod tests {
         scrolled_grid.scroll_display(Scroll::Top);
         assert_eq!(term.grid, scrolled_grid);
     }
+
+    #[test]
+    fn window_title() {
+        let size = SizeInfo {
+            width: 21.0,
+            height: 51.0,
+            cell_width: 3.0,
+            cell_height: 3.0,
+            padding_x: 0.0,
+            padding_y: 0.0,
+            dpr: 1.0,
+        };
+        let mut term = Term::new(&MockConfig::default(), &size, Clipboard::new_nop(), Mock);
+
+        // Title can be set
+        {
+            term.title = "Test".to_string();
+            assert_eq!(term.title, "Test");
+        }
+
+        // Title can be pushed onto stack
+        {
+            term.push_title();
+            term.title = "Next".to_string();
+            assert_eq!(term.title, "Next");
+            assert_eq!(term.title_stack.get(0).unwrap(), "Test");
+        }
+
+        // Title can be popped from stack and set as the window title
+        {
+            term.pop_title();
+            assert_eq!(term.title, "Test");
+            assert!(term.title_stack.is_empty());
+        }
+
+        // Title stack doesn't grow infinitely
+        {
+            for _ in 0..4097 {
+                term.push_title();
+            }
+            assert_eq!(term.title_stack.len(), 4096);
+        }
+
+        // Title and title stack reset when terminal state is reset
+        {
+            term.push_title();
+            term.reset_state();
+            assert_eq!(term.title, "Alacritty");
+            assert!(term.title_stack.is_empty());
+        }
+    }
 }
 
 #[cfg(all(test, feature = "bench"))]
diff --git a/alacritty_terminal/src/tty/windows/conpty.rs b/alacritty_terminal/src/tty/windows/conpty.rs
index fe49b4dc..185acfc2 100644
--- a/alacritty_terminal/src/tty/windows/conpty.rs
+++ b/alacritty_terminal/src/tty/windows/conpty.rs
@@ -143,7 +143,7 @@ pub fn new<'a, C>(
 
     let mut startup_info_ex: STARTUPINFOEXW = Default::default();
 
-    let title = config.window.title.as_ref().map(String::as_str).unwrap_or("Alacritty");
+    let title = config.window.title.clone();
     let title = U16CString::from_str(title).unwrap();
     startup_info_ex.StartupInfo.lpTitle = title.as_ptr() as LPWSTR;