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rxvt-unicode is basically flicker-free withou resorting to
(slow and resource-intensive) double-buffering. the basic
problem is that rxvt clears the screen a few times on each
resize.

note, however, that flickering in general cannot be emoved
with double-buffering, because applications using curses or
slang will clera the screen and redraw on each resize, so it
will only ever be flicker-free outside of full-screen apps.