Squashed Repaints

Basic repaint test for squashed layers. The entire squashing layer should not need repainting when only a portion of it is invalidated. Test interactively by using --show-paint-rects and hovering over elements to change their color.

CASE 1, original layer tree:

CASE 2, overlap1 changes color:

CASE 3, overlap1 and overlap2 change color:

CASE 4, overlap2 and overlap3 change color:

CASE 5, overlap3 and overlap1 change color: