Why does zsh clear to end-of-screen after prompt?

[John Hawkinson <jhawk@alum.mit.edu>]

When I reposition the cursor to the top left corner of the screen (or anywhere), as with ESC [ H, when zsh prints a prompt it clears from the prompt to the end of the screen, oblitering any text on the screen already, which defeats the point of my cursor positioning.

Why does this happen, and why is it desirable?
I can workaround it by setting TERM=dumb, but that has other annoying side effects.
It does not appear to tbe the result of a precmd, and I still see it invoking zsh --no-rcs.

To reproduce:

   seq 1 10; export TERM=xterm-color; echo "\e[H foo\n"

Most of the ten lines output of seq is overwritten and nothing is seen.
By contrast:

  seq 1 10; export TERM=dumb; echo "\e[H foo\n"

prints the ten lines and moves the cursor to the top above them, as expected.

How can I get cursor positining to work without forgoing, e.g., line editing?


I feel like I'm missing something obvious.

--
jhawk@alum.mit.edu
John Hawkinson