gtk: implement unfocused-split opacity and fill

For a long time, us GTK users have been subject to lesser UX by not
knowing which split was focused. Improve the GTK UX by implementing both
unfocused-split-opacity and unfocused-split-fill. This is implemented by
setting the background-color of the notebook stack, and conditionally
applying a new css class "unfocused-split" to the unfocused split.

src/apprt/gtk/App.zig

@@ -70,6 +70,10 @@ x11_xkb: ?x11.Xkb = null,
 /// and initialization was successful.
 transient_cgroup_base: ?[]const u8 = null,
 
+/// True if we have initialized runtime CSS. We only need to do this once for the application, but
+/// we can't perform the intialization until we have created a window
+runtime_css_intialized: bool = false,
+
 pub fn init(core_app: *CoreApp, opts: Options) !App {
     _ = opts;
 

src/apprt/gtk/Surface.zig

@@ -1809,6 +1809,9 @@ fn gtkFocusEnter(_: *c.GtkEventControllerFocus, ud: ?*anyopaque) callconv(.C) vo
     // Notify our IM context
     c.gtk_im_context_focus_in(self.im_context);
 
+    // Unconditionally remove the unfocused split css class
+    c.gtk_widget_remove_css_class(@ptrCast(@alignCast(self.gl_area)), "unfocused-split");
+
     // Notify our surface
     self.core_surface.focusCallback(true) catch |err| {
         log.err("error in focus callback err={}", .{err});
@@ -1823,6 +1826,14 @@ fn gtkFocusLeave(_: *c.GtkEventControllerFocus, ud: ?*anyopaque) callconv(.C) vo
     // Notify our IM context
     c.gtk_im_context_focus_out(self.im_context);
 
+    // We only add the unfocused-split class if we are actually a split
+    switch (self.container) {
+        .split_br,
+        .split_tl,
+        => c.gtk_widget_add_css_class(@ptrCast(@alignCast(self.gl_area)), "unfocused-split"),
+        else => {},
+    }
+
     self.core_surface.focusCallback(false) catch |err| {
         log.err("error in focus callback err={}", .{err});
         return;

src/apprt/gtk/Window.zig

@@ -16,6 +16,7 @@ const input = @import("../../input.zig");
 const CoreSurface = @import("../../Surface.zig");
 
 const App = @import("App.zig");
+const Color = configpkg.Config.Color;
 const Surface = @import("Surface.zig");
 const Tab = @import("Tab.zig");
 const c = @import("c.zig");
@@ -71,6 +72,35 @@ pub fn init(self: *Window, app: *App) !void {
         c.gtk_widget_set_opacity(@ptrCast(window), app.config.@"background-opacity");
     }
 
+    if (!app.runtime_css_intialized) {
+        // Intialize runtime CSS. This CSS requires ghostty configuration values so we don't set it
+        // in style.css. We also have to have a window in order to add a style_context to a display,
+        // so we intialize after creation of our first window
+        app.runtime_css_intialized = true;
+
+        const display = c.gdk_display_get_default();
+        const provider = c.gtk_css_provider_new();
+        defer c.g_object_unref(provider);
+        const fill: Color = app.config.@"unfocused-split-fill" orelse app.config.background;
+        var css_buf: [128]u8 = undefined;
+
+        // We will add the unfocused-split class in our focus callbacks. We unconditionally add the
+        // background-color to the notebook stack because it only comes into play if we have an
+        // unfocused split
+        const css = try std.fmt.bufPrintZ(
+            &css_buf,
+            "widget.unfocused-split {{ opacity: {d:.2}; }}\nstack {{ background-color: rgb({d},{d},{d});}}",
+            .{
+                app.config.@"unfocused-split-opacity",
+                fill.r,
+                fill.g,
+                fill.b,
+            },
+        );
+        c.gtk_css_provider_load_from_string(provider, css);
+        c.gtk_style_context_add_provider_for_display(display, @ptrCast(provider), c.GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+    }
+
     // Use the new GTK4 header bar. We only create a header bar if we have
     // window decorations.
     if (app.config.@"window-decoration") {

src/config/Config.zig

@@ -409,8 +409,6 @@ palette: Palette = .{},
 /// is 0.15. This value still looks weird but you can at least see what's going
 /// on. A value outside of the range 0.15 to 1 will be clamped to the nearest
 /// valid value.
-///
-/// This is only supported on macOS.
 @"unfocused-split-opacity": f64 = 0.7,
 
 /// The color to dim the unfocused split. Unfocused splits are dimmed by
@@ -418,8 +416,6 @@ palette: Palette = .{},
 /// that rectangle and can be used to carefully control the dimming effect.
 ///
 /// This will default to the background color.
-///
-/// This is only supported on macOS.
 @"unfocused-split-fill": ?Color = null,
 
 /// The command to run, usually a shell. If this is not an absolute path, it'll