Use vim master with "huge" build.

1. Invoke insert mode completion with lots of suggestions so the popupmenu covers the entire height of the screen (e g do tag completion based on a single letter only)
2. type <c-r>=<c-f> ( = cmdline window)

Now cmdline window is entered, but it is covered by the popupmenu and unusable. Even after redraw the popupmenu still covers the cmdline window.