I’m observing this behavior where I see WebKit stretching & shrinking a part of page when I scroll Gmail’s vertical preview pane.
Created attachment 178700 [details] screenshot 1
Created attachment 178701 [details] screenshot 2
Probably caused by http://trac.webkit.org/changeset/137215
Created attachment 178707 [details] Patch
http://trac.webkit.org/changeset/137248
This undid the fix for bug 104461. There's a patch there to fix that.