Although I discovered this problem in qtcreator, I believed this is a general QT problem.

If I set QT_SCALE_FACTOR to, say 1.3, and QT_QPA_PLATFORM=wayland, then open qtcreator, any "select box" detects a wrong cursor position. An example of "select box" is "Preference -> Test Editor -> Font -> Size". I have to move my mouse to somewhere lower-right out of the "select box" to select something.

This can be fixed if I set QT_QPA_PLATFORM=xcb.