bug#20847: [display engine] 25.0.50; company-mode popup makes point jump to an entirely different location

[Dmitry Gutov <dgutov@yandex.ru>]

On 06/23/2015 07:39 PM, Eli Zaretskii wrote:

> Would it be possible in this specific case to start the overlay from
> the next line, i.e. leave the newline alone?  I think if you do that,
> things ought to work without any changes.

I suppose.

> Since the "tooltip" pops up
> below this line, it seems to me that the newline which begins the
> overlay now is not needed, is it?

"now"?

If the line after the current has a `display' property, it could still 
lead to the problem like bug#18285. However, `M-x report-emacs-bug' 
buffer (which was the main example for that bug) doesn't have 
particularly long lines, so we could hope the workaround won't be 
triggered there. But anyway, wrong rendering is still better than moving 
point to a different place.

>> The current example shows that it's better to display the cursor on the
>> margin, rather than after the overlay. What are the examples where this
>> is not true?
>
> All the others ;-)

A concrete example would help. The fact that you'd *try to* display the 
cursor at the newline belonging to an overlay display string indicates 
that the overlay must start at that position, doesn't it? Or end.

If it starts earlier, then the cursor might be displayed before or after 
(if wouldn't be displayed in the middle of the overlay string, right?).

> No, I think what makes this case special is the fact that (the visible
> part of) the overlay begins on the line below the one where the user
> types.  So from the user POV, the current line is still occupied only
> by buffer text, and so users will expect the cursor to be displayed as
> if there were no overlay at all.

If I had to pick, I'd probably always display the cursor before such 
overlays, not after. That seems consistent with the logic of expecting 
the cursor "to be displayed as if...".

> By contrast, the "usual" case with overlays that span multiple lines
> is that the situation with positioning the cursor arises when some of
> the overlay string is visible on the current line, and then the user
> expectation is to see the cursor after the string.  That's what the
> code which handles this situation tries to make happen.

If the overlay display string ends at that newline, and point is at the 
end of the overlay, then the display engine exception under the 
discussion will be a no-op. If it ends later, why would we even try to 
display the cursor at that newline in the first place?

>> Even if cursor would look weird in some case, at least point is not
>> forcibly moved to a different position.
>
> We are in agreement regarding the best place of showing the cursor in
> this case.  The only problem is how to do that.

Removing that exception might do the trick. But then, maybe I don't 
really understand what it does.