To test this, one need 2 screens. The following 2 setups were tested to reproduce the issue:
1. 3840x2160@150% + 1920x1200@125%
2. 2880x1800@150% + 1920x1080@100%
Some other combinations that I am not yet aware of may also work. However, for example, "3840x2160@100% + 1920x1200@125%" will not show the bugged behavior. So it seems to be related to certain scaling factor.

But anyway, the steps are:
1. Start Qt Creator on the higher resolution screen. For example, on 3840x2160 one.
2. Open a source code file in code editor.
3. Ctrl + E and then 4 to pop out a separate window for code editor.
4. Drag the popup to the other screen.

Notice that at the moment that popup reaches the other screen, the original code editor in Qt Creator also scales up. Bring popup back, then the original code editor scales back down too.

A video is uploaded. It was shot on a setup of 3 monitors - but that is not necessary. 2 monitors are enough to reproduce the issue. The video is just for illustrating what one can expect.