Disable shadows for transparent windows on macOS

Commit 5725f58 introduced a performance regression on macOS due to
excessive calls to the `invalidateShadow` function, however calling this
function only on redraw after a resize was performed does not fix the
underlying problem.

As a solution, window shadows are now disabled completely for all
transparent windows. This makes sure there is no performance impact,
while still solving the problem with text artifacts on resize.

Fixes #4604.

3 files changed, 15 insertions, 10 deletions

diff --git a/alacritty/src/display.rs b/alacritty/src/display.rs
index fb82b9f8..7fbf0d54 100644
--- a/alacritty/src/display.rs
+++ b/alacritty/src/display.rs
@@ -254,6 +254,10 @@ impl Display {
         #[cfg(target_os = "macos")]
         crossfont::set_font_smoothing(config.ui_config.font.use_thin_strokes);
 
+        // Disable shadows for transparent windows on macOS.
+        #[cfg(target_os = "macos")]
+        window.set_has_shadow(config.ui_config.background_opacity() >= 1.0);
+
         #[cfg(all(feature = "x11", not(any(target_os = "macos", windows))))]
         let is_x11 = event_loop.is_x11();
         #[cfg(not(any(feature = "x11", target_os = "macos", windows)))]
@@ -610,12 +614,6 @@ 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/event.rs b/alacritty/src/event.rs
index 87b1d5c8..bbb0da0f 100644
--- a/alacritty/src/event.rs
+++ b/alacritty/src/event.rs
@@ -1279,6 +1279,10 @@ impl<N: Notify + OnResize> Processor<N> {
         #[cfg(target_os = "macos")]
         crossfont::set_font_smoothing(config.ui_config.font.use_thin_strokes);
 
+        // Disable shadows for transparent windows on macOS.
+        #[cfg(target_os = "macos")]
+        processor.ctx.window.set_has_shadow(config.ui_config.background_opacity() >= 1.0);
+
         *processor.ctx.config = config;
 
         // Update cursor blinking.
diff --git a/alacritty/src/window.rs b/alacritty/src/window.rs
index 1b874d15..418994ed 100644
--- a/alacritty/src/window.rs
+++ b/alacritty/src/window.rs
@@ -32,7 +32,7 @@ use {
 use std::fmt::{self, Display, Formatter};
 
 #[cfg(target_os = "macos")]
-use cocoa::base::id;
+use cocoa::base::{id, NO, YES};
 use glutin::dpi::{PhysicalPosition, PhysicalSize};
 use glutin::event_loop::EventLoop;
 #[cfg(target_os = "macos")]
@@ -441,16 +441,19 @@ impl Window {
         self.windowed_context.resize(size);
     }
 
-    /// Force macOS to clear shadow of transparent windows.
+    /// Disable macOS window shadows.
+    ///
+    /// This prevents rendering artifacts from showing up when the window is transparent.
     #[cfg(target_os = "macos")]
-    pub fn invalidate_shadow(&self) {
+    pub fn set_has_shadow(&self, has_shadows: bool) {
         let raw_window = match self.window().raw_window_handle() {
             RawWindowHandle::MacOS(handle) => handle.ns_window as id,
             _ => return,
         };
 
+        let value = if has_shadows { YES } else { NO };
         unsafe {
-            let _: () = msg_send![raw_window, invalidateShadow];
+            let _: () = msg_send![raw_window, setHasShadow: value];
         }
     }