bug#22873: Can we support multiple cursors?

[Eli Zaretskii]

> Date:  Sun, 13 Aug 2017 11:19:30 -0700
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  22873@debbugs.gnu.org,John Wiegley <jwiegley@gmail.com>,Marcin Borkowski 
> <mbork@mbork.pl>,Richard Stallman <rms@gnu.org>
> 
> I have encountered a situation where the Y and VPOS coordinates returned by 
> `move_it_to` are sometimes out of bounds, and MATRIX_ROW crashes Emacs when 
> drawing fake cursors that are out of bounds.  The solution appears to be a 
> test for whether Y and VPOS are out of bounds.

Actually, the usual solution is to limit move_it_to to
last_visible_y.  Are you saying that you already do that, and the
values of Y and VPOS are still out of bounds?  That would be strange,
because the display engine does that in many places.

> I would like to use `window_box_height` to give me essentially the same value 
> as `window-body-height` with a non-nil PIXELWISE argument.

And that's not what you get?  Can you show a simple example?

`count-screen-lines` is not a viable option because using `vertical-motion` is 
too slow.

It's strange that you say so because vertical-motion uses the same
move_it_to subroutines that you'd like to use directly.

> Testing and limiting the drawing of fake cursors with `y <= window_box_height 
> (w)` gives me mixed results; i.e., sometimes fake cursors are drawn/erased 
> only up to line 49, but sometimes it works up to line 50 (partially visible). 
>  I think I want to test for whether Y is less than or equal to 1000 and if 
> VPOS is less than or equal to 50 -- if the test is true, then go ahead and 
> draw fake cursors up to and including that location.

Are you sure you account for the height of the cursor and the line on
which it is shown?  The Y coordinate is where the line begins; it ends
several pixels lower, i.e. at a greater value of Y.

> However, I can foresee situations where there might be mixed line heights on 
> the visible window, and relying upon the frame character height and window 
> body height will not be reliable.  In a simple case, we can take 1000 divided 
> by 20 and know that the VPOS is 50.

For reliable calculations, you must do everything in pixels.

> Is there a more reliable test for the last partially visible Y and VPOS 
> within the window body height?  Should I be using pos_visible_p in some 
> capacity here?

You could use pos_visible_p, although it, again, uses move_it_to.  But
I don't think I have a clear understanding of your problem, so maybe
it's immaterial.