As the graph below, it is pretty obvious that the desiger does not correctly show the real size of the actural window. It was also too big (even bigger than the entire screen) when I create a new project.

My DPI scaling is 150%.

note:

I think it was added up twice, in the window itself and the widget designer, because the preview is just roughly 1.5 times larger than the actural window. This is definitely a bug, and almost made the designer unusable.

I haven't tested on other platforms. I would assume this bug also exists on Linux.