bug#19188: point adjustemnt moves *into* invisible text

[Jonas Bernoulli]

Sorry for the delay.

Stefan Monnier <monnier@IRO.UMontreal.CA> writes:

> The fact that point is equal to 3 means that point is *between*
> character 2 and character 3.  So it's not *inside* an invisible text,
> but is right at the boundary.

I understand that and should have chosen my words more carefully.

I noticed that the behavior was not consistent and jumped to the
conclusion that the invisibility property did receive some special
handling but that there was a bug in how that was done.  Turns out it is
supposed to behave according to its stickyness, like other properties.

It is however possible to get the behavior that I want.  This requires
that `invisible's stickyness is set accordingly.

  (push (cons 'invisible t) text-property-default-nonsticky)

For modes with collapsible section (magit, org-mode ...) this behavior
makes a lot of sense.  Basically this teaches Emacs to define
`invisible' as "cannot be seen by the eye", which happens to be a
reasonable definition I would think :-)

Also required is this hack which relies on `get-text-property' and
`get-pos-property' disagreeing about the value of `invisible'.

  (add-hook 'post-command-hook #'magit-post-command-adjust-point t t)

  (defun magit-post-command-adjust-point ()
    (when (and (get-text-property (point) 'invisible)
               (not (if (fboundp 'get-pos-property) ; since 24.4, see #1671
                        (get-pos-property (point) 'invisible)
                      (get-text-property (1+ (point)) 'invisible))))
      (goto-char (next-single-char-property-change (point) 'invisible))))

Is there a better way?

I am not (no longer) convinced that Emacs should necessarily behave the
way I thought it was supposed to; not by default at least.  But I do
think that in some cases it feels strange to adjust point, just because
point is "property-invisible" and even though the following character
were the cursor would be displayed is "visible-to-the-eye".  It is
especially confusing when only point is adjusted and the cursor stays
were it would have been without the adjustment of point.

So maybe, just maybe, it might make sense if "property-visible-point"
and "eye-visible-cursor" would agree by default.




>
> The position 5 (i.e. between character 4 and character 5) is at the
> other end of the boundary.
>
> The reason why Emacs decided to put point at position 3 rather than
> leave it at position 5 is because the boundary at position 3 is "less
> invisible" than the boundary at position 5.
> You can check it with
>
>   M-: (list (get-pos-property 3 'invisible) (get-pos-property 5 'invisible)) 
> RET
>
> This is because text-properties by default are front-non-sticky and
> rear-sticky, so if point is at position 5 and you type a character, that
> character will inherit the invisible property, whereas if you're at
> position 3 and you type a character this character will not inherit the
> invisible property.
>
> If you want point to be at position 5 rather than position 3, then you
> need to change the front/rear-stickiness of this invisible
> property accordingly.
>
>> When point adjustment is disabled (non-nil disable-point-adjustment or
>> global-disable-point-adjustment) then this does not happen.
>
> I assume you know why ;-)
>
>> It also does not happen when moving forward, e.g. starting at "1"
>> C-p C-f places the cursor on "5" *and* point is also 5.
>
> C-p C-f doesn't do it for me (it doesn't even reach the invisible part of the
> text), and if I change the recipe to C-f C-f it doesn't work either
> (point stays at position 3).
>
> But indeed C-n gets me to position 5, which is wrong (and doing M-: (point)
> returns 5 but moves me to position 3, so doing it again returns 3 :-( ).
> So we do have a bug here.
>
>
>         Stefan