zsh's approach is clever and doesn't require bidirectional communication. They always print their inverted "%" and then they print as many spaces as you have columns in the terminal (minus one). If the last process ended with a CRLF, they will now be in the last column of the same line; otherwise they will have wrapped around to a new line (assuming the terminal is in auto wrap mode, which is the default). Then they print a carriage return and the prompt as normal.

This does break if the terminal has auto wrap turned off (echo -e "\033[?7l"), but otherwise it's very clever.

That solution sounds even more hacky. Just like I expected resizing my iterm2 window causes glitches because it's text reflowing. I love that feature, but I realize this isn't a problem for all terminals.

looks interesting, but it doesn't seem to be compatible with either my setup in general (bash 3.2.25 inside screen inside PuTTY) or my current prompt, which comes out of a shell function and embeds the number of files in pwd, username@hostname:pwd, and previous status, with coloring applied to various parts.

specifically your function just sort of sits there doing nothing when i try it....