If the WebView is layer backed (via -setWantsLayer:YES), then there are issues with the rendering of composited content. It draws twice.
<rdar://problem/7916580>
Created attachment 71358 [details] Patch
Comment on attachment 71358 [details] Patch View in context: https://bugs.webkit.org/attachment.cgi?id=71358&action=review > WebKit/mac/WebView/WebFrame.mm:532 > +- (BOOL)_showFlattenedCompositingLayers:(CGContextRef)context This name is no good. This method doesn’t show anything. > WebKit/mac/WebView/WebFrame.mm:540 > + if (WKCGContextIsBitmapContext(context)) { Can reverse the test and return early here.
Created attachment 71371 [details] Patch
http://trac.webkit.org/changeset/70199