WebGLLayerChromium and impl use the accelerated canvas "backbuffer" directly. In threaded compositing mode, this causes partial webgl results to appear onscreen. Related issue: https://bugs.webkit.org/show_bug.cgi?id=72738
jamesr has fixed these problems in Bug 80459 and related fixes (thanks!). Closing as a duplicate. *** This bug has been marked as a duplicate of bug 80459 ***