This is Qt 6.6.1 backport that runs on Windows 7 (what?). The repository contains patched source files from the qtbase module. 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 can use our prebuild binaries (we used Visual C++ 2019 with OpenSSL 1.1.1k statically linked, see compile_win.pl script) or compile Qt yourself.

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: