Strange appearance when Qt::CustomizeWindowHint is enabled

Running the "Window Flags Example", enabling Qt::CustomizeWindowHint will cause the window has a thin white bar on the top.

Only tested on Windows. Don't know what it looks like on other platforms.

Re-reproducible on Qt 5.13.x and 5.14.x, earlier versions not tested.

I'm not good at English, maybe the screenshot in the appendix can describe this bug better.

DPI: 125% (but the device pixel ratio of this widget is 1.0, not 1.25)

I can't place any widgets or paint anything on it, I feel like it's some kind of left over of the hidden title bar. I have no idea of the root cause, but if it's not a painting issue, then it should be a window system problem.