Call glFinish right after swap_buffers on X11

On X11 `swap_buffers` does not block for vsync. However the next OpenGl command
will block to synchronize (this is `glClear` in Alacritty), which causes a
permanent one frame delay.

Calling `glFinish` after swapping buffers forces Alacritty to finish the buffer
swap before returning control to the event loop.

Fixes #3061.

3 files changed, 32 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cbe1ab17..4534989b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -30,6 +30,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Tabstops not being reset with `reset`
 - Selection not cleared when switching between main and alt grid
 - Fallback to `LC_CTYPE=UTF-8` on macOS without valid system locale
+- Resize lag on launch under some X11 wms
+- Increased input latency due to vsync behavior on X11
 
 ## 0.4.2
 
diff --git a/alacritty/src/display.rs b/alacritty/src/display.rs
index eafab8c2..980b0ea0 100644
--- a/alacritty/src/display.rs
+++ b/alacritty/src/display.rs
@@ -118,6 +118,8 @@ pub struct Display {
     renderer: QuadRenderer,
     glyph_cache: GlyphCache,
     meter: Meter,
+    #[cfg(not(any(target_os = "macos", windows)))]
+    is_x11: bool,
 }
 
 impl Display {
@@ -198,14 +200,20 @@ impl Display {
             api.clear(background_color);
         });
 
+        #[cfg(not(any(target_os = "macos", windows)))]
+        let is_x11 = event_loop.is_x11();
+
         // We should call `clear` when window is offscreen, so when `window.show()` happens it
         // would be with background color instead of uninitialized surface.
         #[cfg(not(any(target_os = "macos", windows)))]
         {
             // On Wayland we can safely ignore this call, since the window isn't visible until you
             // actually draw something into it.
-            if event_loop.is_x11() {
-                window.swap_buffers()
+            if is_x11 {
+                window.swap_buffers();
+                renderer.with_api(&config, &size_info, |api| {
+                    api.finish();
+                });
             }
         }
 
@@ -237,6 +245,8 @@ impl Display {
             size_info,
             urls: Urls::new(),
             highlighted_url: None,
+            #[cfg(not(any(target_os = "macos", windows)))]
+            is_x11,
         })
     }
 
@@ -499,6 +509,18 @@ impl Display {
         }
 
         self.window.swap_buffers();
+
+        #[cfg(not(any(target_os = "macos", windows)))]
+        {
+            if self.is_x11 {
+                // On X11 `swap_buffers` does not block for vsync. However the next OpenGl command
+                // will block to synchronize (this is `glClear` in Alacritty), which causes a
+                // permanent one frame delay.
+                self.renderer.with_api(&config, &size_info, |api| {
+                    api.finish();
+                });
+            }
+        }
     }
 }
 
diff --git a/alacritty/src/renderer/mod.rs b/alacritty/src/renderer/mod.rs
index 764b5ec5..f62e6f8d 100644
--- a/alacritty/src/renderer/mod.rs
+++ b/alacritty/src/renderer/mod.rs
@@ -923,6 +923,12 @@ impl<'a, C> RenderApi<'a, C> {
         }
     }
 
+    pub fn finish(&self) {
+        unsafe {
+            gl::Finish();
+        }
+    }
+
     fn render_batch(&mut self) {
         unsafe {
             gl::BufferSubData(