bug#28936: move_it_in_display_line_to returns MOVE_POS_MATCH_OR_ZV before ZV

[Eli Zaretskii]

> Date: Sat, 21 Oct 2017 20:02:58 -0700
> From: Keith David Bershatsky <esq@lawlist.com>
> 
> 1.  `move_it_in_display_line' could use an additional comment at the outset 
> to let readers know that it is not compatible with moving to X in a 
> horizontal scrolling and/or truncate lines situation.  The existing 
> commentary that it was intended for external use was insufficient to deter 
> me, and it required a learning curve on my part to better understand its 
> limited potential use.

More commentary can never do any harm, but the incompatibility you
describe is news to me.  Can you show a recipe in "emacs -Q" that
would demonstrate this issue?  Or show a complete code snippet,
starting with start_display or init_iterator, where this issue
happens?

> 2.  When IT is on the last line in the buffer containing a few or more 
> characters, `move_it_in_display_line_to' stops short of the target X and 
> erroneously returns MOVE_POS_MATCH_OR_ZV when used as follows.  I have 
> display-line-numbers set to a non-nil value in the event that makes a 
> difference.  There is nothing special in terms of text-properties or overlays 
> present in the buffer.
> 
>     int target_x = [Some arbitrary X that is a few characters before ZV.];
> 
>     move_it_in_display_line_to (it, ZV, target_x, MOVE_TO_POS | MOVE_TO_X);

You don't show how the value of 'it' was set up before this call, so
it's hard to look into this issue.  I can only tell that I've reviewed
all the code paths that lead to MOVE_POS_MATCH_OR_ZV being returned,
and they all seem to be conditioned either on reaching the position or
hitting ZV.  Maybe I missed something, but without a complete code
snippet that's the best I could do.

> The workaround is to compare the result of MOVE_POS_MATCH_OR_ZV with 
> IT_CHARPOS to ensure that we are really at a ZV situation.

MOVE_POS_MATCH_OR_ZV doesn't necessarily mean you are at ZV, you could
also be at POS or after it.