[UI-side compositing] Fix 2-up shrink mode to work with UI-side compositing
<rdar://problem/103373463>
Pull request: https://github.com/WebKit/WebKit/pull/8272
Committed 258565@main (f4154cb024ec): <https://commits.webkit.org/258565@main> Reviewed commits have been landed. Closing PR #8272 and removing active labels.