WebKitTestRunner's window snapshots are pure white no matter what's on screen when they're taken, if tiled drawing is on (possibly any time remote layer hosting is occurring?) and WKTR's window has a buffered backing store.
This doesn't show up in ref tests because it's broken in both the ref and the actual result; pixel tests make it the most clear (but we don't have any tiled-drawing pixel tests, nor do we run the other ones very often).
I've added a mismatch test to ensure this doesn't happen again, as I'm going to switch WKTR to tiled drawing 24/7 very soon, and it would be really bad if we lost all ref image coverage :|
Conveniently, we're really mostly interested in testing unbuffered windows anyway.
Created attachment 212789 [details]
I'm seeing some tests that use the non-tiled path fail with this change, so it might require more investigation.
We can't override the window resolution with an unbuffered window, so this breaks the workaround we use to make tests work on Retina machines. I'll try to come up with something else.
Created attachment 212837 [details]
Re-opened since this is blocked by bug 122035
Re-opened since this is blocked by bug 122176
Meh, I'm just folding this into https://bugs.webkit.org/show_bug.cgi?id=121859
*** This bug has been marked as a duplicate of bug 121859 ***