…d r=gw

composite_simple() calculates combined dirty rect from all tile's dirty rect. But the combined dirty rect becomes invalid when there is an old tile that was dropped. The dropped tile's dirty rect is not counted in composite_simple().

Differential Revision: https://phabricator.services.mozilla.com/D62531

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/8e08016b3d0acf0c5c3116b087d5455df573d99c