Fix flickering during resize on Wayland

This also fixes an issue of windows not being rendered while resizing.

Fixes #6069.

7 files changed, 81 insertions, 34 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 69d28921..46a9d77b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -38,6 +38,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Bottom gap for certain builtin box drawing characters
 - Incorrect built-in glyphs for `U+2567` and `U+2568`
 - Character mappings in the DEC special graphics character set (line drawing)
+- Window flickering on resize on Wayland
 
 ## 0.10.1
 
diff --git a/alacritty/src/cli.rs b/alacritty/src/cli.rs
index 3c64458b..83b317b9 100644
--- a/alacritty/src/cli.rs
+++ b/alacritty/src/cli.rs
@@ -237,7 +237,7 @@ pub struct WindowIdentity {
 }
 
 impl WindowIdentity {
-    /// Override the [`WindowIdentityConfig`]'s fields with the [`WindowOptions`].
+    /// Override the [`WindowIdentity`]'s fields with the [`WindowOptions`].
     pub fn override_identity_config(&self, identity: &mut Identity) {
         if let Some(title) = &self.title {
             identity.title = title.clone();
diff --git a/alacritty/src/display/mod.rs b/alacritty/src/display/mod.rs
index eb7b8d53..99df511f 100644
--- a/alacritty/src/display/mod.rs
+++ b/alacritty/src/display/mod.rs
@@ -359,6 +359,9 @@ pub struct Display {
     /// Unprocessed display updates.
     pub pending_update: DisplayUpdate,
 
+    /// The renderer update that takes place only once before the actual rendering.
+    pub pending_renderer_update: Option<RendererUpdate>,
+
     is_damage_supported: bool,
     debug_damage: bool,
     damage_rects: Vec<DamageRect>,
@@ -367,6 +370,19 @@ pub struct Display {
     meter: Meter,
 }
 
+/// Pending renderer updates.
+///
+/// All renderer updates are cached to be applied just before rendering, to avoid platform-specific
+/// rendering issues.
+#[derive(Debug, Default, Copy, Clone)]
+pub struct RendererUpdate {
+    /// Should resize the window.
+    resize: bool,
+
+    /// Clear font caches.
+    clear_font_cache: bool,
+}
+
 impl Display {
     pub fn new<E>(
         config: &UiConfig,
@@ -425,7 +441,7 @@ impl Display {
         // If the scaling factor changed update the glyph cache and mark for resize.
         let should_resize = (estimated_scale_factor - window.scale_factor).abs() > f64::EPSILON;
         let (cell_width, cell_height) = if should_resize {
-            Self::update_glyph_cache(&mut renderer, &mut glyph_cache, scale_factor, config, font)
+            Self::update_font_size(&mut glyph_cache, scale_factor, config, font)
         } else {
             (cell_width, cell_height)
         };
@@ -433,7 +449,7 @@ impl Display {
         // Load font common glyphs to accelerate rendering.
         debug!("Filling glyph cache with common glyphs");
         renderer.with_loader(|mut api| {
-            glyph_cache.load_common_glyphs(&mut api);
+            glyph_cache.reset_glyph_cache(&mut api);
         });
 
         if let Some(dimensions) = dimensions.filter(|_| should_resize) {
@@ -520,6 +536,7 @@ impl Display {
             visual_bell: VisualBell::from(&config.bell),
             colors: List::from(&config.colors),
             pending_update: Default::default(),
+            pending_renderer_update: Default::default(),
             is_damage_supported,
             debug_damage,
             damage_rects,
@@ -529,30 +546,31 @@ impl Display {
     /// Update font size and cell dimensions.
     ///
     /// This will return a tuple of the cell width and height.
-    fn update_glyph_cache(
-        renderer: &mut Renderer,
+    fn update_font_size(
         glyph_cache: &mut GlyphCache,
         scale_factor: f64,
         config: &UiConfig,
         font: &Font,
     ) -> (f32, f32) {
-        renderer.with_loader(|mut api| {
-            let _ = glyph_cache.update_font_size(font, scale_factor, &mut api);
-        });
+        let _ = glyph_cache.update_font_size(font, scale_factor);
 
         // Compute new cell sizes.
         compute_cell_size(config, &glyph_cache.font_metrics())
     }
 
-    /// Clear glyph cache.
-    fn clear_glyph_cache(&mut self) {
+    /// Reset glyph cache.
+    fn reset_glyph_cache(&mut self) {
         let cache = &mut self.glyph_cache;
         self.renderer.with_loader(|mut api| {
-            cache.clear_glyph_cache(&mut api);
+            cache.reset_glyph_cache(&mut api);
         });
     }
 
     /// Process update events.
+    ///
+    /// XXX: this function must not call to any `OpenGL` related tasks. Only logical update
+    /// of the state is being performed here. Rendering update takes part right before the
+    /// actual rendering.
     pub fn handle_update<T>(
         &mut self,
         terminal: &mut Term<T>,
@@ -568,31 +586,29 @@ impl Display {
         let (mut cell_width, mut cell_height) =
             (self.size_info.cell_width(), self.size_info.cell_height());
 
-        // Ensure we're modifying the correct OpenGL context.
-        self.window.make_current();
+        if pending_update.font().is_some() || pending_update.cursor_dirty() {
+            let renderer_update = self.pending_renderer_update.get_or_insert(Default::default());
+            renderer_update.clear_font_cache = true
+        }
 
         // Update font size and cell dimensions.
         if let Some(font) = pending_update.font() {
             let scale_factor = self.window.scale_factor;
-            let cell_dimensions = Self::update_glyph_cache(
-                &mut self.renderer,
-                &mut self.glyph_cache,
-                scale_factor,
-                config,
-                font,
-            );
+            let cell_dimensions =
+                Self::update_font_size(&mut self.glyph_cache, scale_factor, config, font);
             cell_width = cell_dimensions.0;
             cell_height = cell_dimensions.1;
 
             info!("Cell size: {} x {}", cell_width, cell_height);
-        } else if pending_update.cursor_dirty() {
-            self.clear_glyph_cache();
         }
 
         let (mut width, mut height) = (self.size_info.width(), self.size_info.height());
         if let Some(dimensions) = pending_update.dimensions() {
             width = dimensions.width as f32;
             height = dimensions.height as f32;
+
+            let renderer_update = self.pending_renderer_update.get_or_insert(Default::default());
+            renderer_update.resize = true
         }
 
         let padding = config.window.padding(self.window.scale_factor);
@@ -618,10 +634,36 @@ impl Display {
 
         // Resize terminal.
         terminal.resize(self.size_info);
+    }
+
+    /// Update the state of the renderer.
+    ///
+    /// NOTE: The update to the renderer is split from the display update on purpose, since
+    /// on some platforms, like Wayland, resize and other OpenGL operations must be performed
+    /// right before rendering, otherwise they could lock the back buffer resulting in
+    /// rendering with the buffer of old size.
+    ///
+    /// This also resolves any flickering, since the resize is now synced with frame callbacks.
+    pub fn process_renderer_update(&mut self) {
+        let renderer_update = match self.pending_renderer_update.take() {
+            Some(renderer_update) => renderer_update,
+            _ => return,
+        };
 
         // Resize renderer.
-        let physical = PhysicalSize::new(self.size_info.width() as _, self.size_info.height() as _);
-        self.window.resize(physical);
+        if renderer_update.resize {
+            let physical =
+                PhysicalSize::new(self.size_info.width() as _, self.size_info.height() as _);
+            self.window.resize(physical);
+        }
+
+        // Ensure we're modifying the correct OpenGL context.
+        self.window.make_current();
+
+        if renderer_update.clear_font_cache {
+            self.reset_glyph_cache();
+        }
+
         self.renderer.resize(&self.size_info);
 
         if self.collect_damage() {
diff --git a/alacritty/src/renderer/text/gles2.rs b/alacritty/src/renderer/text/gles2.rs
index 32aaa173..01470813 100644
--- a/alacritty/src/renderer/text/gles2.rs
+++ b/alacritty/src/renderer/text/gles2.rs
@@ -463,7 +463,7 @@ pub struct TextShaderProgram {
     ///
     /// If GL_EXT_blend_func_extended is not available, the rendering is split into 4 passes.
     /// One is used for the background and the rest to perform subpixel text rendering according to
-    /// https://github.com/servo/webrender/blob/master/webrender/doc/text-rendering.md.
+    /// <https://github.com/servo/webrender/blob/master/webrender/doc/text-rendering.md>.
     ///
     /// Rendering is split into three passes.
     u_rendering_pass: GLint,
diff --git a/alacritty/src/renderer/text/glyph_cache.rs b/alacritty/src/renderer/text/glyph_cache.rs
index b57b10b7..72415900 100644
--- a/alacritty/src/renderer/text/glyph_cache.rs
+++ b/alacritty/src/renderer/text/glyph_cache.rs
@@ -264,19 +264,22 @@ impl GlyphCache {
         loader.load_glyph(&glyph)
     }
 
-    /// Clear currently cached data in both GL and the registry.
-    pub fn clear_glyph_cache<L: LoadGlyph>(&mut self, loader: &mut L) {
+    /// Reset currently cached data in both GL and the registry to default state.
+    pub fn reset_glyph_cache<L: LoadGlyph>(&mut self, loader: &mut L) {
         loader.clear();
-        self.cache = HashMap::default();
+        self.cache = Default::default();
 
         self.load_common_glyphs(loader);
     }
 
-    pub fn update_font_size<L: LoadGlyph>(
+    /// Update the inner font size.
+    ///
+    /// NOTE: To reload the renderers's fonts [`Self::reset_glyph_cache`] should be called
+    /// afterwards.
+    pub fn update_font_size(
         &mut self,
         font: &Font,
         scale_factor: f64,
-        loader: &mut L,
     ) -> Result<(), crossfont::Error> {
         // Update dpi scaling.
         self.rasterizer.update_dpr(scale_factor as f32);
@@ -304,8 +307,6 @@ impl GlyphCache {
         self.metrics = metrics;
         self.builtin_box_drawing = font.builtin_box_drawing;
 
-        self.clear_glyph_cache(loader);
-
         Ok(())
     }
 
diff --git a/alacritty/src/window_context.rs b/alacritty/src/window_context.rs
index 6ead6e4e..f04a8e04 100644
--- a/alacritty/src/window_context.rs
+++ b/alacritty/src/window_context.rs
@@ -325,6 +325,9 @@ impl WindowContext {
         }
 
         if self.dirty {
+            // Force the display to process any pending display update.
+            self.display.process_renderer_update();
+
             self.dirty = false;
 
             // Request immediate re-draw if visual bell animation is not finished yet.
diff --git a/alacritty_terminal/src/term/color.rs b/alacritty_terminal/src/term/color.rs
index a9494e7a..1cfdec6b 100644
--- a/alacritty_terminal/src/term/color.rs
+++ b/alacritty_terminal/src/term/color.rs
@@ -20,8 +20,8 @@ pub struct Rgb {
 }
 
 impl Rgb {
-    /// Implementation of W3C's luminance algorithm:
-    /// https://www.w3.org/TR/WCAG20/#relativeluminancedef
+    /// Implementation of W3C's luminance
+    /// [algorithm](https://www.w3.org/TR/WCAG20/#relativeluminancedef)
     fn luminance(self) -> f64 {
         let channel_luminance = |channel| {
             let channel = channel as f64 / 255.;