Using multiple displays with different resolution and DPI seems to cause problems with QT_USE_NATIVE_WINDOWS=1. Without it, everything seems fine with the same setup.
I could only reproduce this with Qt Creator. QT_USE_NATIVE_WINDOWS=1 is used by Squish for testing. This has worked with older Qt Creator versions, but now with 4.10, it somehow causes window content to be resized even though the whole window is maximized. Resizing the window also does not help.