Resizing on the left or top edges looks horrible. It is very visible with render-to-texture widgets (say QQuickWidget) but even the simplest raster widget app exhibits the same issue.

Launch qtbase/examples/widgets/mainwindow/application and increase the size of the window by dragging left/up on the left/top edge. Then watch how sluggishly the UI repaints, allowing empty areas to become visible during the resize.

NB the same issue is present on Linux with GL stuff, havne't verified with plain widgets.