Yeah that's an issue with the browser export, no clue why it happens but there's quite a lot of jank with that version in general lol
(edit: I have found out that it doesn't happen if you select borderless or exclusive fullscreen in the game's settings, it just happens if you fullscreen it on itch, woops)
Your idea with the blocks is actually pretty great, and now I regret not having thought of that 48 hours ago