add theme titlebar height and app-owned strip dragging

examples/gallery/src/main.rs

@@ -2435,6 +2435,9 @@ fn main() {
                     appears_transparent: true,
                     ..Default::default()
                 }),
+                // The strips move the window themselves, so a tab dragged inside
+                // the titlebar band never drags the window with it.
+                app_owns_titlebar_drag: true,
                 ..Default::default()
             },
             |window, cx| {