Before a window is filled, it is shown on the screen with decoration (titlebar, border) and transparent uninitialized background. This is a visual glitch when any window is shown.

To make it easier to see I slowed down paint events in the attached example. You can step the attached video frame-by-frame to see when the window is animated-in that the background is  a copy of what was under it.

You can see from the debug log there are several redundant paint events that seem to be ignored before the background is finally shown.