This is because the last window size changes, it fills the space that
was just before used for typing the command line.
You should be able to see what happens when 'writedelay' is set to a
small number, e.g. 10. Does (part of) the screen clear, or is it drawn
over the existing text? It looks like it is cleared, perhaps that can be
avoided... Hmm, a comment says:
// Redraw is needed for command line completion
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