Yep, thought that might play a part too, but I have (off camera) tested it by putting it at a slant against my bed frame. Same issue if it isn't being held.
I should clarify I was wrong about it only glitching on my bed and carpet. The reason for this was that if the device is plugged in, it won't show the problem, and previously every time it was on the table it was plugged in.
EDIT: Could people reading this try to open up your browser and attempt to pinch zoom into the page while leaving it on a surface unplugged? When you pinch zoom, do it so your fingers are directly over one and other with the screen in portrait, so that a line drawn between your fingers is parallel with the long side of the screen.
This causes glitches for me as well.