QCombobox poppup position is wrong when using High DPI and Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor

Took me a while to isolate the issue into a MCVE, but I finally figured it out and hosted a very short code sample with explanations and images in the README here: https://github.com/jmarrec/QtTestbed/tree/main/Test-Combobox-Popup-Issue

Here's the bottom line:

1. Combobox popup position issue

• Windows 10 (21H2) (Mac is not affected)
• High DPI display
• Set Display Scaling to 250% (fractional)
• Use `QApplicationn:setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor)`
• Move the main window to the left of the screen: popup is ok, it aligns (see Image *left.png*)
• Move the main window to the right of the screen: it gets an offset that gets worse and worse (see *right.png*)

• Now try with 200% scaling: it works fine (see *right_200pct.png*)

Note:

• if I start the application at 200%, then switch to 250% while it's running, then it works fine
• If I use `Qt::HighDpiScaleFactorRoundingPolicy::PassThrough` at 250% it doesn't exhibit the issue