Subject: Re: bug#18504: 24.3.93; posn-at-point intermittently returns wrong value for a valid buffer position

From: Dmitry Gutov <[email-masked]>
Date: Fri, 19 Sep 2014 17:29:31 +0400

On 09/19/2014 05:12 PM, Eli Zaretskii wrote:

>> Wouldn't setting a conditional breakpoint help?
>
> Conditional breakpoint where?

In the "call a function" routine, maybe? If the function is 
"posn-at-point", and if its returned X coord is zero, halt!

Doing it in Elisp doesn't seem to produce the desired result, because 
edebug doesn't exactly stop Emacs in place.

> With these issues, the only way to find the bug is to step through the
> offending code _before_ it produces the wrong values.  So we need a
> condition that happens at the beginning of that code, not where it
> produces the final wrong results.

If `posn-at-point' is itself stateless (and doesn't modify any relevant 
caches), why not call it again at that point?