Fun redraw issue with double-width characters

[Mikael Magnusson <mikachu@xxxxxxxxx>]

I lied about the "fun".

I got a random thought, "what if i enter five doublewidth characters,
then go back to the start and enter a bunch of spaces". Turns out
doing that causes the line to clear and cursor to move up one line.
Doesn't happen with four or less. Long story short, it happens only if
ich1 is defined in the terminfo for the current $TERM, which it
happens to be for rxvt-unicode, but also for rxvt. It doesn't matter
if you're using xterm or rxvt or urxvt, just set TERM=rxvt, enter five
ã and go back to the start and hold down space. I don't really know
where to begin debugging this. I checked strace output and these lines
are what zsh does when there are two, one and zero cells remaining on
the line, respectively (note that the redraw issue happens not on
wrap, but the step before).

write(10, " \343\201\276\343\201\276\343\201\276\343\201\276\343\201\276\33[10D",
21) = 21
write(10, "\33[@ \33[10C\33[K\33[10D", 17) = 17
write(10, "\33[@ \33[9C \r\33[K\33[A\33[84C", 21) = 21

If i remove the ich1 entry from terminfo (it's the \33[@ bit), i don't
have the problem.

write(10, " \343\201\276\343\201\276\343\201\276\343\201\276\343\201\276\33[10D",
21) = 21
write(10, " \343\201\276\343\201\276\343\201\276\343\201\276\343\201\276\33[10D",
21) = 21
write(10, " \343\201\276\343\201\276\343\201\276\343\201\276\343\201\276\33[K\33[10D",
24) = 24

-- 
Mikael Magnusson