bug#56561: 29.0.50; Infloop in try_window

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

> On second thought, I take this back.  I don't see how we could have a
> blank tooltip.  We call try_window just so we could compute the size
> of the text in it, which then allows us to know the size of the
> tooltip.  The situation where nrows_scale_factor is increased happens
> when we get to the bottom of the window, which for a tooltip means we
> already laid out all the text and are just producing empty glyph rows
> beyond the end of the text.  ncols_scale_factor could theoretically
> happen before we reach the end of the text, but I'd like to see
> something like that happening before I believe it; and even if it does
> happen in the very first line, the tooltip will not be empty, just
> truncated.

No, we call try_window to generate the window contents, and then call
update_single_window followed by flush_frame to immediately flush the
contents to display.

This is because redisplay actually cannot run by itself in some cases
where we do want tooltips to be displayed.

> We could add an assertion to verify that try_window gets to ZV in this
> case before it returns, if we want to be able to detect those cases.

This could still work.  I think `TRY_WINDOW_IGNORE_FONTS_CHANGE' means
the caller isn't ready for try_window to fail.