GTK Fix unfocused-split-fill (#8813)

Attempts a resolution for
https://github.com/ghostty-org/ghostty/discussions/8572

This matches the behavior of the old GTK apprt where
unfocused-split-fill /opacity doesn't apply when there is only one
active surface.

src/apprt/gtk/class/split_tree.zig

@@ -112,6 +112,25 @@ pub const SplitTree = extern struct {
                 },
             );
         };
+
+        pub const @"is-split" = struct {
+            pub const name = "is-split";
+            const impl = gobject.ext.defineProperty(
+                name,
+                Self,
+                bool,
+                .{
+                    .default = false,
+                    .accessor = gobject.ext.typedAccessor(
+                        Self,
+                        bool,
+                        .{
+                            .getter = getIsSplit,
+                        },
+                    ),
+                },
+            );
+        };
     };
 
     pub const signals = struct {
@@ -210,6 +229,14 @@ pub const SplitTree = extern struct {
             }
         }
 
+        // Bind is-split property for new surface
+        _ = self.as(gobject.Object).bindProperty(
+            "is-split",
+            surface.as(gobject.Object),
+            "is-split",
+            .{ .sync_create = true },
+        );
+
         // Create our tree
         var single_tree = try Surface.Tree.init(alloc, surface);
         defer single_tree.deinit();
@@ -511,6 +538,18 @@ pub const SplitTree = extern struct {
         ));
     }
 
+    fn getIsSplit(self: *Self) bool {
+        const tree: *const Surface.Tree = self.private().tree orelse &.empty;
+        if (tree.isEmpty()) return false;
+
+        const root_handle: Surface.Tree.Node.Handle = .root;
+        const root = tree.nodes[root_handle.idx()];
+        return switch (root) {
+            .leaf => false,
+            .split => true,
+        };
+    }
+
     //---------------------------------------------------------------
     // Virtual methods
 
@@ -816,6 +855,9 @@ pub const SplitTree = extern struct {
             v.grabFocus();
         }
 
+        // Our split status may have changed
+        self.as(gobject.Object).notifyByPspec(properties.@"is-split".impl.param_spec);
+
         // Our active surface may have changed
         self.as(gobject.Object).notifyByPspec(properties.@"active-surface".impl.param_spec);
 
@@ -873,6 +915,7 @@ pub const SplitTree = extern struct {
                 properties.@"has-surfaces".impl,
                 properties.@"is-zoomed".impl,
                 properties.tree.impl,
+                properties.@"is-split".impl,
             });
 
             // Bindings

src/apprt/gtk/class/surface.zig

@@ -275,6 +275,24 @@ pub const Surface = extern struct {
                 },
             );
         };
+
+        pub const @"is-split" = struct {
+            pub const name = "is-split";
+            const impl = gobject.ext.defineProperty(
+                name,
+                Self,
+                bool,
+                .{
+                    .default = false,
+                    .accessor = gobject.ext.privateFieldAccessor(
+                        Self,
+                        Private,
+                        &Private.offset,
+                        "is_split",
+                    ),
+                },
+            );
+        };
     };
 
     pub const signals = struct {
@@ -503,6 +521,10 @@ pub const Surface = extern struct {
         /// A weak reference to an inspector window.
         inspector: ?*InspectorWindow = null,
 
+        // True if the current surface is a split, this is used to apply
+        // unfocused-split-* options
+        is_split: bool = false,
+
         // Template binds
         child_exited_overlay: *ChildExited,
         context_menu: *gtk.PopoverMenu,
@@ -601,6 +623,16 @@ pub const Surface = extern struct {
         return @intFromBool(config.@"bell-features".border);
     }
 
+    /// Callback used to determine whether unfocused-split-fill / unfocused-split-opacity
+    /// should be applied to the surface
+    fn closureShouldUnfocusedSplitBeShown(
+        _: *Self,
+        focused: c_int,
+        is_split: c_int,
+    ) callconv(.c) c_int {
+        return @intFromBool(focused == 0 and is_split != 0);
+    }
+
     pub fn toggleFullscreen(self: *Self) void {
         signals.@"toggle-fullscreen".impl.emit(
             self,
@@ -2829,6 +2861,7 @@ pub const Surface = extern struct {
             class.bindTemplateCallback("notify_mouse_shape", &propMouseShape);
             class.bindTemplateCallback("notify_bell_ringing", &propBellRinging);
             class.bindTemplateCallback("should_border_be_shown", &closureShouldBorderBeShown);
+            class.bindTemplateCallback("should_unfocused_split_be_shown", &closureShouldUnfocusedSplitBeShown);
 
             // Properties
             gobject.ext.registerProperties(class, &.{
@@ -2847,6 +2880,7 @@ pub const Surface = extern struct {
                 properties.title.impl,
                 properties.@"title-override".impl,
                 properties.zoom.impl,
+                properties.@"is-split".impl,
             });
 
             // Signals

src/apprt/gtk/ui/1.2/surface.blp

@@ -115,6 +115,20 @@ Overlay terminal_page {
     label: bind template.mouse-hover-url;
   }
 
+  [overlay]
+  // Apply unfocused-split-fill and unfocused-split-opacity to current surface
+  // this is only applied when a tab has more than one surface
+  Revealer {
+    reveal-child: bind $should_unfocused_split_be_shown(template.focused, template.is-split) as <bool>;
+    transition-duration: 0;
+
+    DrawingArea {
+      styles [
+        "unfocused-split",
+      ]
+    }
+  }
+
   // Event controllers for interactivity
   EventControllerFocus {
     enter => $focus_enter();