In the presence of multiple, horizontal splits, the org-todo popup
consumes ~50% of the vertical real-estate of its northern neighbor when
created. This is due to an edge case with some failsafe code somewhere
down org-fit-window-to-buffer's stack. This fix ensures the popup is
always a sane size.
And by enabling +popup-buffer-mode in it, we ensure it will be treated
as a popup (e.g. solaire-mode will be active in it, no modeline, etc).
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