I have the same problem, but only with one particular database. Something got messed up in the forms I guess.
Initial workarounds:
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Menu> Window> Close window will close the window.
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Alt-middle mouse click seems to at least allow me to minimize or close the windows.
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Right clicking the system tray also allowed me to move them to my non-primary (other) screen. (This didn’t work at first. I had to try it a bunch of times to get it done.)
Solution:
I happen to have two monitors, a smaller on on my laptop and a larger one hooked up w/ HDMI. I shutdown and rebooted.
When I restarted the database with the issues, and re-opened one of the broken forms, I noticed that while it was loading (it’s a large complicated form that takes a few seconds to load), that even thought the form was loading on my notebook screen, some of it was overlaying my larger screen to the right. Also at this point in time the minimize, maximize and close buttons were showing!
So I was able to quickly drag the corner of the window that was in the process of loading, and make it much smaller and this ended up fixing the broken database.
Now the buttons are back and seem to work correctly.
I suspect there might be a bug in LO or something it relies on having to do with two screens of different sizes.
If your database or calc sheet isn’t slow enough, then edit it somehow and make it load slower. I’ll let you figure out the best way to do this. Also you might be able to give it a very low priority or something like that to buy you time.
Good luck. It was an uncomfortable issue while it lasted.
Using LO base 5.4 and 6.0.1 on GNU/Linux Debian 9.4 &Cinnamon.