So I am still running into this quite frequently. The way it typically goes is I click on a another tab and the window stops updating until either enough time has passed (multiple seconds), or I press alt-tab or resize the window. During these "freezes" when I move the cursor I can see it change when hoovering areas where there will be text after things go back to normal.
Bug 1575171 Comment 8 Edit History
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So I am still running into this quite frequently. The way it typically goes is I click on a another tab and the window stops updating until either enough time has passed (multiple seconds), or I press alt-tab or resize the window. During these "freezes" when I move the cursor I can see it change when hoovering areas where there will be text after things go back to normal. Edit: to be clear this doesn't only happen upon tab switching. It might only happen after I click something although I don't know for sure. I don't remember the window stopping to update halfway through scrolling. I had the profiler running while it happened once and none of the threads involved with JS or gfx or inputs were busy during the freeze.