gtk: add unfocused_widget when split created from menu

When a split is created from a menu action, the focus is lost before the
split is made which prevents the surface from having the
unfocused_widget. Move the logic to add the unfocused_widget to the
overlay to an exported function which is called when the split is
created.

src/apprt/gtk/Split.zig

@@ -77,6 +77,7 @@ pub fn init(
         .parent = &sibling.core_surface,
     });
     errdefer surface.destroy(alloc);
+    sibling.dimSurface();
 
     // Create the actual GTKPaned, attach the proper children.
     const orientation: c_uint = switch (direction) {

src/apprt/gtk/Surface.zig

@@ -1836,16 +1836,11 @@ 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 widget if we are actually a split
+    // We only dim the surface if we are a split
     switch (self.container) {
         .split_br,
         .split_tl,
-        => blk: {
+        => self.dimSurface(),
-            if (self.unfocused_widget != null) break :blk;
-            self.unfocused_widget = c.gtk_drawing_area_new();
-            c.gtk_widget_add_css_class(self.unfocused_widget.?, "unfocused-split");
-            c.gtk_overlay_add_overlay(self.overlay, self.unfocused_widget.?);
-        },
         else => {},
     }
 
@@ -1855,6 +1850,15 @@ fn gtkFocusLeave(_: *c.GtkEventControllerFocus, ud: ?*anyopaque) callconv(.C) vo
     };
 }
 
+/// Adds the unfocused_widget to the overlay. If the unfocused_widget has already been added, this
+/// is a no-op
+pub fn dimSurface(self: *Surface) void {
+    if (self.unfocused_widget != null) return;
+    self.unfocused_widget = c.gtk_drawing_area_new();
+    c.gtk_widget_add_css_class(self.unfocused_widget.?, "unfocused-split");
+    c.gtk_overlay_add_overlay(self.overlay, self.unfocused_widget.?);
+}
+
 fn gtkCloseConfirmation(
     alert: *c.GtkMessageDialog,
     response: c.gint,