Eli Zaretskii wrote:

> Does this problem go away if you enlarge the terminal window such that
> the entire File and Tools menus can be displayed without overlaying
> the mode line? 

I maximized the xterm before starting Emacs, and still very quickly got
into the state where there is a duplicate "--" off to the right of the
real menu (same as my "1.png" from earlier). It does seem less common
with a bigger terminal though.

> Does this happen if you use the menu "reasonably", i.e. without
> leaning on arrow keys? 

So far no, IME.

> Does it happen if you use C-p instead of the down arrow?

Leaning on C-p produces glitches, yes. Here's a fun one: