Fix artifacts on macOS with transparent windows

Due to the way macOS draws shadows for transparent windows, resizing
them will lead to text artifacts remaining present after a window has
been resized.

The `invalidateShadow` call is used whenever the opacity isn't `1.0` to
make sure these shadows are cleared before redrawing, so no artifacts
remain when resizing transparent windows.

Fixes #889.

5 files changed, 30 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ae6deaea..d9701053 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -42,6 +42,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - High CPU usage on BSD with live config reload enabled
 - Alacritty not discarding invalid escape sequences starting with ESC
 - Crash due to clipboard not being properly released on Wayland
+- Shadow artifacts when resizing transparent windows on macOS
 
 ### Removed
 
diff --git a/Cargo.lock b/Cargo.lock
index f3aa5acd..f9b53890 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -29,6 +29,7 @@ dependencies = [
  "alacritty_terminal",
  "bitflags",
  "clap",
+ "cocoa 0.24.0",
  "copypasta",
  "crossfont",
  "dirs",
@@ -42,6 +43,7 @@ dependencies = [
  "objc",
  "parking_lot",
  "png",
+ "raw-window-handle",
  "serde",
  "serde_json",
  "serde_yaml",
diff --git a/alacritty/Cargo.toml b/alacritty/Cargo.toml
index 6e833f34..4810ced4 100644
--- a/alacritty/Cargo.toml
+++ b/alacritty/Cargo.toml
@@ -46,6 +46,8 @@ xdg = "2"
 png = { version = "0.16.8", default-features = false, optional = true }
 
 [target.'cfg(target_os = "macos")'.dependencies]
+raw-window-handle = "0.3.3"
+cocoa = "0.24.0"
 objc = "0.2.2"
 
 [target.'cfg(not(any(target_os="windows", target_os="macos")))'.dependencies]
diff --git a/alacritty/src/display.rs b/alacritty/src/display.rs
index 1fe1d2de..fb82b9f8 100644
--- a/alacritty/src/display.rs
+++ b/alacritty/src/display.rs
@@ -610,6 +610,12 @@ impl Display {
         #[cfg(all(feature = "wayland", not(any(target_os = "macos", windows))))]
         self.request_frame(&self.window);
 
+        // Clear window shadows to prevent shadow artifacts on macOS.
+        #[cfg(target_os = "macos")]
+        if config.ui_config.background_opacity() < 1.0 {
+            self.window.invalidate_shadow();
+        }
+
         self.window.swap_buffers();
 
         #[cfg(all(feature = "x11", not(any(target_os = "macos", windows))))]
diff --git a/alacritty/src/window.rs b/alacritty/src/window.rs
index f4a38183..1b874d15 100644
--- a/alacritty/src/window.rs
+++ b/alacritty/src/window.rs
@@ -31,6 +31,8 @@ use {
 
 use std::fmt::{self, Display, Formatter};
 
+#[cfg(target_os = "macos")]
+use cocoa::base::id;
 use glutin::dpi::{PhysicalPosition, PhysicalSize};
 use glutin::event_loop::EventLoop;
 #[cfg(target_os = "macos")]
@@ -41,6 +43,10 @@ use glutin::window::{
     CursorIcon, Fullscreen, UserAttentionType, Window as GlutinWindow, WindowBuilder, WindowId,
 };
 use glutin::{self, ContextBuilder, PossiblyCurrent, WindowedContext};
+#[cfg(target_os = "macos")]
+use objc::{msg_send, sel, sel_impl};
+#[cfg(target_os = "macos")]
+use raw_window_handle::{HasRawWindowHandle, RawWindowHandle};
 #[cfg(windows)]
 use winapi::shared::minwindef::WORD;
 
@@ -435,6 +441,19 @@ impl Window {
         self.windowed_context.resize(size);
     }
 
+    /// Force macOS to clear shadow of transparent windows.
+    #[cfg(target_os = "macos")]
+    pub fn invalidate_shadow(&self) {
+        let raw_window = match self.window().raw_window_handle() {
+            RawWindowHandle::MacOS(handle) => handle.ns_window as id,
+            _ => return,
+        };
+
+        unsafe {
+            let _: () = msg_send![raw_window, invalidateShadow];
+        }
+    }
+
     fn window(&self) -> &GlutinWindow {
         self.windowed_context.window()
     }