Generate a frame when a transaction scrolls

This no longer happens automatically in WebRender, so we must do it
manually.

Fixes #20762.

components/compositing/compositor.rs

@@ -937,6 +937,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             let cursor = webrender_api::WorldPoint::from_untyped(&cursor);
             let mut txn = webrender_api::Transaction::new();
             txn.scroll(scroll_location, cursor);
+            txn.generate_frame();
             self.webrender_api.send_transaction(self.webrender_document, txn);
             self.waiting_for_results_of_scroll = true
         }