We're seeing crashes on macOS inside GraphicsContextGL::reshape(). Specifically when we submit work at the end of the function via glFlush.
At the moment the cause is a mystery, because we should bail out before then if the multisample renderbuffer was not complete. In the hope that it helps somewhat, add a call to glGetError to double check that there isn't anything horribly wrong before we talk to the GPU.
Created attachment 396812 [details]
Committed r260301: <https://trac.webkit.org/changeset/260301>