Overlay broken on Linux Mint 21.1 (with standard Cinnamon desktop)

[Letiliel]

YomiNinja looks like exactly what I need, but I'm having several breaking issues:

1. When hovering over overlay rectangles, the OCR'd text glitches, either showing the correct text or another to the right. Please check the provided video (the image glitching at the top is a bug with the casting app).
2. The overlay is misplaced when applied to a window. It's fine when full screen but that doesn't solve issue 1.
3. The external border of the overlay is missing the bottom and right lines, preventing resizing it when misplaced.
4. When launching YomiNinja the desktop panels (menu bar etc) are hidden.

I would like to sponsor fixes for these. Or at least the first one which is the only one really preventing me to use your app. If you're interested, let's discuss

https://github.com/matt-m-o/YomiNinja/assets/4447503/379bee4b-6236-4eee-be99-52a88f1f9106

I tried changing various options and disabling extensions.