gtk(wayland): add support for background blur on KDE Plasma

build.zig

@@ -1479,7 +1479,14 @@ fn addDeps(
 
                     const wayland = b.createModule(.{ .root_source_file = scanner.result });
 
+                    const plasma_wayland_protocols = b.dependency("plasma_wayland_protocols", .{
+                        .target = target,
+                        .optimize = optimize,
+                    });
+                    scanner.addCustomProtocol(plasma_wayland_protocols.path("src/protocols/blur.xml"));
+
                     scanner.generate("wl_compositor", 1);
+                    scanner.generate("org_kde_kwin_blur_manager", 1);
 
                     step.root_module.addImport("wayland", wayland);
                     step.linkSystemLibrary2("wayland-client", dynamic_link_opts);

src/apprt/gtk/App.zig

@@ -847,9 +847,11 @@ fn configChange(
     new_config: *const Config,
 ) void {
     switch (target) {
-        // We don't do anything for surface config change events. There
+        .surface => |surface| {
-        // is nothing to sync with regards to a surface today.
+            if (surface.rt_surface.container.window()) |window| window.syncAppearance(new_config) catch |err| {
-        .surface => {},
+                log.warn("error syncing appearance changes to window err={}", .{err});
+            };
+        },
 
         .app => {
             // We clone (to take ownership) and update our configuration.

src/apprt/gtk/Window.zig

@@ -392,6 +392,17 @@ pub fn init(self: *Window, app: *App) !void {
     c.gtk_widget_show(window);
 }
 
+/// Updates appearance based on config settings. Will be called once upon window
+/// realization, and every time the config is reloaded.
+///
+/// TODO: Many of the initial style settings in `create` could possibly be made
+/// reactive by moving them here.
+pub fn syncAppearance(self: *Window, config: *const configpkg.Config) !void {
+    if (self.wayland) |*wl| {
+        try wl.setBlur(config.@"background-blur-radius" > 0);
+    }
+}
+
 /// Sets up the GTK actions for the window scope. Actions are how GTK handles
 /// menus and such. The menu is defined in App.zig but the action is defined
 /// here. The string name binds them.
@@ -565,6 +576,10 @@ fn gtkRealize(v: *c.GtkWindow, ud: ?*anyopaque) callconv(.C) bool {
         self.wayland = wayland.SurfaceState.init(v, wl);
     }
 
+    self.syncAppearance(&self.app.config) catch |err| {
+        log.err("failed to initialize appearance={}", .{err});
+    };
+
     return true;
 }
 

src/apprt/gtk/wayland.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const c = @import("c.zig").c;
 const wayland = @import("wayland");
 const wl = wayland.client.wl;
+const org = wayland.client.org;
 const build_options = @import("build_options");
 
 const log = std.log.scoped(.gtk_wayland);
@@ -9,6 +10,7 @@ const log = std.log.scoped(.gtk_wayland);
 /// Wayland state that contains application-wide Wayland objects (e.g. wl_display).
 pub const AppState = struct {
     display: *wl.Display,
+    blur_manager: ?*org.KdeKwinBlurManager = null,
 
     pub fn init(display: ?*c.GdkDisplay) ?AppState {
         if (comptime !build_options.wayland) return null;
@@ -45,6 +47,9 @@ pub const SurfaceState = struct {
     app_state: *AppState,
     surface: *wl.Surface,
 
+    /// A token that, when present, indicates that the window is blurred.
+    blur_token: ?*org.KdeKwinBlur = null,
+
     pub fn init(window: *c.GtkWindow, app_state: *AppState) ?SurfaceState {
         if (comptime !build_options.wayland) return null;
 
@@ -66,6 +71,32 @@ pub const SurfaceState = struct {
     }
 
     pub fn deinit(self: *SurfaceState) void {
+        if (self.blur_token) |blur| blur.release();
+    }
+
+    pub fn setBlur(self: *SurfaceState, blurred: bool) !void {
+        log.debug("setting blur={}", .{blurred});
+
+        const mgr = self.app_state.blur_manager orelse {
+            log.warn("can't set blur: org_kde_kwin_blur_manager protocol unavailable", .{});
+            return;
+        };
+
+        if (self.blur_token) |blur| {
+            // Only release token when transitioning from blurred -> not blurred
+            if (!blurred) {
+                mgr.unset(self.surface);
+                blur.release();
+                self.blur_token = null;
+            }
+        } else {
+            // Only acquire token when transitioning from not blurred -> blurred
+            if (blurred) {
+                const blur_token = try mgr.create(self.surface);
+                blur_token.commit();
+                self.blur_token = blur_token;
+            }
+        }
     }
 };
 
@@ -73,6 +104,10 @@ fn registryListener(registry: *wl.Registry, event: wl.Registry.Event, state: *Ap
     switch (event) {
         .global => |global| {
             log.debug("got global interface={s}", .{global.interface});
+            if (bindInterface(org.KdeKwinBlurManager, registry, global, 1)) |iface| {
+                state.blur_manager = iface;
+                return;
+            }
         },
         .global_remove => {},
     }

src/config/Config.zig

@@ -583,11 +583,27 @@ palette: Palette = .{},
 @"background-opacity": f64 = 1.0,
 
 /// A positive value enables blurring of the background when background-opacity
-/// is less than 1. The value is the blur radius to apply. A value of 20
+/// is less than 1.
+///
+/// On macOS, the value is the blur radius to apply. A value of 20
 /// is reasonable for a good looking blur. Higher values will cause strange
 /// rendering issues as well as performance issues.
 ///
-/// This is only supported on macOS.
+/// On KDE Plasma under Wayland, the exact value is _ignored_ —  the reason is
+/// that KWin, the window compositor powering Plasma, only has one global blur
+/// setting and does not allow applications to have individual blur settings.
+///
+/// To configure KWin's global blur setting, open System Settings and go to
+/// "Apps & Windows" > "Window Management" > "Desktop Effects" and select the
+/// "Blur" plugin. If disabled, enable it by ticking the checkbox to the left.
+/// Then click on the "Configure" button and there will be two sliders that
+/// allow you to set background blur and noise strengths for all apps,
+/// including Ghostty.
+///
+/// All other Linux desktop environments are as of now unsupported. Users may
+/// need to set environment-specific settings and/or install third-party plugins
+/// in order to support background blur, as there isn't a unified interface for
+/// doing so.
 @"background-blur-radius": u8 = 0,
 
 /// The opacity level (opposite of transparency) of an unfocused split.