>     Most likely this is caused by garbage collection, and
>     displaying tooltips makes GC to occur sooner than when tooltips are
>     disabled.
>
> I don't follow the connection.  What does GC have to do with garbling
> tooltips?

Displaying the tooltip frame while a menu is displayed requires some Lisp
consing, so GC starts sooner and it garbles menu items while menus are
still displayed.

> Are you using a mode in which the tooltip appears in the echo area?

This bug can be reproduced even when the tooltip appears in the echo area.
I guess displaying the tooltip in the echo area requires less consing,
so GC activates later than when the tooltip appears in the separate window.

So the bug is not in tooltips, but in GC which garbles menu items.

I can reproduce this bug only on non-toolkit builds.

-- 
Juri Linkov
http://www.jurta.org/emacs/