This is Qt 6.6.1 backport that runs on Windows 7 (what?). The repository is the qtbase module source code with all required modifications already applied. Approach is based on this forum thread but better: many improvements amongst important fallbacks to default Qt 6 behaviour when running on newer Windows.

You basically need to compile Qt yourself (we use the compile_win script).

The only issue noticed is that scalled UI is somewhat too big for 125% scalling option set in Windows 7. Something should be tweaked with dpi settings probably.

Qt 6.6 designer running on Windows 7: