I wonder why this happens and how the context menu appears because I know it doesn't update the entire screen and only the area under it when it disappears. I'm guessing that could be a reason.
I can't reproduce this in any of the browsers I'm using (Chrome, Firefox, Safari, all on a Mac). However, Chrome v85 and v86 had a bad Canvas bug that resulted in all kinds of mis-coloring and clipping bugs. If this is in Chrome you might want to update to the latest v87 that got out today, and see if this issue still happens to you.