bug#11094: Wrong cursor positioning with display+invisible

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> > For some value of "correctly".  E.g., position the cursor over the
>> > ".", which is the first file name in the archive, and type "C-x =".
>> > You will see "63", which is a lie: the actual value of point is 111.
>> I don't think it's a lie because that text is preceded by invisible
>> text, so while "." is at 111, point is indeed at 63 (and that's just
>> because of adjust_point_for_property, so we can get point to stay at
>> 111 by being more careful with text property's stickiness).
> Having point report X when it is position on a character whose
> position is Y, or jump when you move it from one character of a file
> name to the next violates the principle of least astonishment, IMO.

That's an inevitable consequence of using `invisible'.  So yes, I agree
that `invisible' should be used with great care, but there is no bug in
this particular part (the underlying problem is that the block cursor is
drawn *on* characters whereas point is *between* characters).

>> the cursor is drawn at the right place.  I think this is the core of the
>> problem that's handled differently from Emacs-23.
>> [ IIUC you've gotten to the same conclusion.  ]
> While there could be more than one way to cut this cake, I still think
> we should encourage Lips programmers to use the `cursor' property in
> these situations.

Hmm... I haven't actually tried to use the `cursor' property, but I'm
not sure it would help when point is actually outside of the text area
covered by any of the display strings.

> Should we close this bug, then?

We might as well keep it open until the problem is actually solved.


        Stefan