Created attachment 252085 [details]
Use the pageExtendedBackgroundColor for the background color of the WKView,
if we have one (and aren't using a transparent background). This means
during asynchronous resize or other cases where the Web process isn't keeping
up with the UI process during a resize, we'll see background color instead of white.
Attachment 252085 [details] did not pass style-queue:
ERROR: Source/WebKit2/ChangeLog:1: ChangeLog entry has no bug number [changelog/bugnumber] 
Total errors found: 1 in 2 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Re-opened since this is blocked by bug 145326