Windows 10: Window stuck in aways-on-top mode

Version: Win10 Edu, 2004, x64

I have the unusual problem that sporadically a window gets stuck in always-on-top mode and there is no way to get it back to normal behavior except restarting the program.

I don't really know how to reliably reproduce it and I only noticed this behavior recently when I started regularly using the "split-screen" window arrangement on a single monitor - that is, if you drag a window to the left or right edge of the screen, it's maximized to fill only half the screen and then you can select another open window to fill the other half. Afterwards, you can drag the separator between the two windows left or right to adjust where the screen is split (if you want other ratios than 50:50).

Usually I arrange two Firefox windows in this mode or one Firefox window and one Media Player Classic window and Firefox would be stuck always on top after some time. The thing is that it doesn't happen right away (or else I could easily reproduce it) but after some cycles of shuffeling windows around (maximizing, minimizing, moving, split screen etc.) and maybe with some hibernation cycles in between. It's hard to pin-point when it gets stuck but eventually it does because the Firefox application is typically never closed until a reboot. The stuck-on-top problem goes away when the window is closed and re-opened but I want to avoid having to do that to my main browser window with many open tabs. I thought this problem may be specific to Firefox but then I somehow managed to get a Notepad window stuck on top as well, so it seems like a weird Windows problem.

Here are some "features" of this stuck-on-top mode that I observed:

- It can affect more than one window of one application (e.g. 2 Firefox windows can be stuck).
- It does not necessarily affect ALL windows of an application: If I open a new Firefox window while having one (or more) Firefox windows stuck on top, the new Firefox window will behave normally and be always behind the stuck windows (unless the stuck ones are minimized). Since Firefox is multi-instance now, I can't tell if this really applies to two windows of the same instance.
- Stuck-on-top windows behave as expected for always-on-top windows, sharing one "z-layer" with other always-on-top windows. That means a stuck window with focus will be on top of other windows in the stuck state. A different application with a built-in intentional always-on-top mode (like a media player) will also cover stuck windows when it has focus.
- The "split-screen" mode behaves a bit differently with stuck windows: When you drag it to the screen edge, it will still fill one half of the screen as expected, but it doesn't show the preview of the other windows to pick a second one to fill the other side. This also happens with windows that were set intentionally to always-on-top (e.g. media player), so I assume it's a "normal" behavior with any always-on-top windows.

I understand it's a very rare issue and I only could find advice for Windows 7 where this could be solved with the Ctrl + Alt + ESC shortcut, but this shortcut doesn't help on Windows 10. The stuck window loses focus but it's still on top.

Any ideas what could have caused this and how to avoid it? Does anyone know some kind of window management tool (maybe even a built-in one) that can read/set the always-on-top state?

:)