During the first flush for a layer, CoordinatedGraphicsLayer::updateContentBuffers() renders the contents of the tiles required to cover the visible area. After that, it requests a new flush to render the required tiles that are not inside the visible area yet (but belong to the cover area). But this flush request is performed with a direct call to nofityFlushRequired(), and that's discarded because we are already on a layer flush, so the remaining tiles are not rendered unless a new layer flush happens because of some other component.
Created attachment 366704 [details]
Created attachment 366705 [details]
Created attachment 366706 [details]
Comment on attachment 366706 [details]
Clearing flags on attachment: 366706
Committed r243866: <https://trac.webkit.org/changeset/243866>
All reviewed patches have been landed. Closing bug.