Geoff, many thanks for your report about the working Big Sur version.
There are no open questions from it.
Concerning the status line glitch, there is already an issue report on GitHub.
This is related with auto save, the status bar is toggled for this message and forces a layout rebuild.
It can disturb drawing, when it happens just on mouse input, but not more.
I can't say, why your command line has black background. I've not heard that before and it is surely not intended by any newer version.
Is the command box the only input field which is affected or are there others too?
Maybe it's somehow related with a dark desktop schema?
Or have you played around with Qt style sheets? (see https://github.com/LibreCAD/LibreCAD/wiki/Style-Sheets
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