It could be an ANGLE change responsible. I know Apple's been working on the Metal rendering backend. Maybe something in that is breaking for you.
I'm not sure if the video represents what's going on perfectly, but it appears like maybe it's reading old data from an improperly cleared framebuffer or something?
I'm not sure if the video represents what's going on perfectly, but it appears like maybe it's reading old data from an improperly cleared framebuffer or something?