Normal rendering, no high DPI scaling involved. Freetype is not affected.
Notably shows in Qt Creator 'File changed on disk' dialog.
Appears to be Vista style only. This is then related to the QWindowsVistaStyle::drawPrimitive() drawing a white theme panel on message boxes at src\widgets\styles\qwindowsvistastyle.cpp:788 and caused by some alpha blending issue.
Introduced by "Cleanup rgb32 text-blending code path" d160809015e49d6435dc876135b88f17af4f8008 https://codereview.qt-project.org/#/c/184708/