Occasionally, all of the UI at the top of the Firefox window is replaced by blank white space when the window is maximized. (See screenshot 1.) When the window is restored (non-maximized), it looks normal. Other applications' windows look normal. This has happened occasionally over the last few months (at least before Ff 69 and Win 10 1903). I have no idea what triggers it. When this happens, restarting Windows is the only way to restore normal operation. The problem persists even if I: restore & maximize, minimize & maximize, restart Ff, restart Ff in safe mode, update & restart Ff, or make a fresh profile (See screenshot 2.). Sometimes, the blank area also includes a clipping of the UI as it appeared before maximizing. (See screenshot 2.) Clicking in the white area interacts with the desktop, not Firefox. I can open folders, drag icons, etc. I can't find any questions or bugs about this particular behavior. Any help is appreciated.