Regression in http://trac.webkit.org/changeset/138987
On windows PlatformContextSkia::m_drawingToImageBuffer still needs to be set when painting form controls on transparent layers. It is used as a hint to properly blend the bitmap painted using GDI, which does not handle the alpha channel properly.
Created attachment 191326 [details]
Committed r144688: <http://trac.webkit.org/changeset/144688>