Currently the WebGL content is not in the HTMLCanvasElement image buffer. When that buffer is read and we have a WebGL canvas, we need to use readPixels to get the contents and put them into the image buffer and then return that.
This was mostly implemented with https://bugs.webkit.org/show_bug.cgi?id=34719. But it now crashes on Mac, which is described in https://bugs.webkit.org/show_bug.cgi?id=42862
This isn't fixed yet.
This bug is not "fixed", even though a (failed) implementation was done. I'm closing this on in favor of https://bugs.webkit.org/show_bug.cgi?id=42862. I'm making it a dup to keep the connection. *** This bug has been marked as a duplicate of bug 42862 ***