Based on some quick research into this, the windows are likely jumping sizes because of something like a stuck key, a shortcut, the hot‑corner, the graphics driver, or the compositor being triggered unintentionally. I'd say you should first test for any stuck keys that could be the cause (IE: Super, Alt + F10/Space) with xev – multiple events = sticky key. Or you could just plug in an external keyboard and if it stops, the built‑in keyboard is at fault.

If stuck keys aren’t the issue, try turning off hot corners + window tiling in the settings, then restart the device. If it's still happening after this, checking that you’re using the correct graphics driver would be the move. You can run inxi -Gx to adjust the driver.

If none of this fixes the issue, peep the recent updates in /var/log/apt/history.log for any packages which were recently updated that could cause the problem. You could also make a fresh user account to see if the problem is system‑wide or confined to your config. Once you find and correct the trigger, the flickering and whatnot will stop, you just have to figure out what's causing it.

Is your F11 key jammed or something?