unicode in prompt breaks command search

[Nathan Sidwell <nathan@xxxxxxx>]

For reasons, I put a multibyte unicode char in $PROMPT, this ends up 
messing with zsh's idea of the current column, as it doesn't understand 
the display width of $PROMPT.

For example reverse search, here's an example using <f0><9f><a1><85>

set PROMPT="🡅"

press C-r, see cursor gets indented another 3 spaces -- we're now at 
column 5, which would be correct if each byte of $PROMPT was a separate 
display char.

A slightly modified example is to enter some text before pressing C-r, 
then the first 3 chars get retained on the display (but not the input 
buffer at least).

Is this supposed to work?

This is ZSH 5.2 on a fedora 30 system, and an XFCE terminal.  Let me 
know if you need more info (I'm not on the list).

nathan
--
Nathan Sidwell