gtk: rebind is-split after moving split cross-tree

It turns out we never unbound the split from its original tree after
moving, which means `is-split` in particular is desynced and leads to
hilarious artifacts like how `unfocused-split-*` options just stop
working properly. I only realized this is a thing after the naïve
drag handle config option didn't work properly. Fun!

src/apprt/gtk/class/split_tree.zig

@@ -239,12 +239,7 @@ 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 },
-        );
+        surface.bindIsSplit(self);
 
         // Create our tree
         var single_tree = try Surface.Tree.init(alloc, surface);
@@ -452,6 +447,9 @@ pub const SplitTree = extern struct {
             // Finally, set the final tree structures for both tree widgets
             source_tree_widget.setTree(&new_source_tree);
             self.setTree(&after_split);
+
+            // Re-bind vital properties like `is-split`
+            source.bindIsSplit(self);
         }
     }
 

src/apprt/gtk/class/surface.zig

@@ -691,6 +691,7 @@ pub const Surface = extern struct {
         // True if the current surface is a split, this is used to apply
         // unfocused-split-* options
         is_split: bool = false,
+        is_split_binding: ?*gobject.Binding = null,
 
         action_group: ?*gio.SimpleActionGroup = null,
 
@@ -847,6 +848,18 @@ pub const Surface = extern struct {
         return @intFromBool(config.@"bell-features".border);
     }
 
+    pub fn bindIsSplit(self: *Self, tree: *SplitTree) void {
+        const priv = self.private();
+        if (priv.is_split_binding) |bind| bind.unbind();
+
+        priv.is_split_binding = tree.as(gobject.Object).bindProperty(
+            "is-split",
+            self.as(gobject.Object),
+            "is-split",
+            .{ .sync_create = true },
+        );
+    }
+
     /// Callback used to determine whether unfocused-split-fill / unfocused-split-opacity
     /// should be applied to the surface
     fn closureShouldUnfocusedSplitBeShown(