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KevinH - Thanks for checking all that.
For me at least, an EnvVar for drawing a coloured border around the control that has focus is sufficient. If you make it a Preference setting someone is bound to ask for a shortcut, an on/off toggle in a menu, and a toolbar button
As for the Text cursor issues (to me a caret is a ^), I'm happy to leave it be.
I don't understand why so many applications respect the Windows text cursor width setting in single line text boxes - where I think it's least needed, but disregard it in multi-line text text boxes - where I think it's most needed. It's not a Qt specific thing.
Back when 'expert systems' were the go, I had something that had width and colour settings for its text cursor - would have been on OS/2 or Apple Lisa.
BR