Borderless parent window falsely makes child dialog borderless too

Reproducer attached. Noted that the issue cannot be reproduced by 6.6.1, so I guess some kind of fix exists and was merged already. So whatever the fix is, please backport it to 6.5.* since 6.5 is LTS version.

Steps to reproduce:
1. Run
2. Maximize
3. Click "Click Me"
4. Click "Pick Screen Color"
5. Click on the main widget's background to pick its color
6. Click on the main widget's background again to change focus and trigger the bug
Upon bug being triggered, the child color dialog becomes borderless too.