Auto merge of #20766 - mrobinson:fix-scrolling-issue, r=gw3583

Generate a frame when a transaction scrolls

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

Fixes #20762.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #20762.

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because Servo does not currently test scrolling in a reliable way.

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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
         }