T20: on Wayland the window asks for ext_background_effect_v1 on its surface, through a guest connection on gpui's display; one region wider than any window, which the compositor clips, comes and goes with Blurred; the log names the way blur goes; a wayland-server compositor tests it, and scripts/linux.sh runs the tests on Linux in a container

src/blur.rs

@@ -2,6 +2,9 @@ use gpui::{Window, WindowBackgroundAppearance};
 
 use crate::settings::Settings;
 
+#[cfg(target_os = "linux")]
+mod wayland;
+
 /// What the window shows of the desktop: blurred or clear while the background lets it through.
 pub fn appearance(settings: &Settings) -> WindowBackgroundAppearance {
     if settings.background_opacity >= 1.0 {
@@ -13,13 +16,39 @@ pub fn appearance(settings: &Settings) -> WindowBackgroundAppearance {
     }
 }
 
-/// Sets `window` to what the settings ask when it differs from `shown`; logs the way taken.
-pub fn follow(window: &Window, shown: &mut WindowBackgroundAppearance, settings: &Settings) {
-    let wanted = appearance(settings);
-    if wanted != *shown {
-        *shown = wanted;
+/// What the main window shows of the desktop, as last set, and on Wayland the blur behind its surface.
+pub struct Blur {
+    shown: WindowBackgroundAppearance,
+    #[cfg(target_os = "linux")]
+    wayland: Option<wayland::Effect>,
+}
+
+impl Blur {
+    /// The window as `WindowOptions` opened it from these settings.
+    pub fn new(window: &Window, settings: &Settings) -> Self {
+        let shown = appearance(settings);
+        #[cfg(not(target_os = "linux"))]
+        let _ = window;
+        Self {
+            shown,
+            #[cfg(target_os = "linux")]
+            wayland: wayland::Effect::on(window, shown == WindowBackgroundAppearance::Blurred),
+        }
+    }
+
+    /// Sets the window to what the settings ask when it differs from what it shows; logs the way taken.
+    pub fn follow(&mut self, window: &Window, settings: &Settings) {
+        let wanted = appearance(settings);
+        if wanted == self.shown {
+            return;
+        }
+        self.shown = wanted;
         window.set_background_appearance(wanted);
         log::info!("blur: the window is {wanted:?} on {}", std::env::consts::OS);
+        #[cfg(target_os = "linux")]
+        if let Some(effect) = &self.wayland {
+            effect.blur(wanted == WindowBackgroundAppearance::Blurred);
+        }
     }
 }