bug#27427: 26.0.50; Native line numbers lead to display error in company-mode popup

[Dmitry Gutov <dgutov@yandex.ru>]

On 6/22/17 5:55 PM, Eli Zaretskii wrote:

>> Yep! And there are zero outstanding bug reports related to this.
> 
> Well, except this one ;-)

But it's not about line-prefix, it's about a new feature in Emacs that 
was just introduced in a way that's different from those that came before.

>> I'm saying it's not easy, and I'm not brimming with ideas.
> 
> Is it not easy because the assumption about column-zero is hard-coded
> in many places?  Or for some other reason?

Because if we're not allowed to use column-zero, the first line of the 
popup and the rest are behaving differently. And, depending on which 
feature is causing the first character of the visual line to be on a 
non-zero column, the popup lines are offset in different directions.

> Keeping the numbers out of the margins was my explicit design goal,
> because some packages want the margins, and we don't have a good
> solution for "sharing" margins.

It's not without problems, though, as it introduces a new, different 
type of margin, doesn't it? Why not put it outside of the window bounds 
next to the "actual" margins? That's one option.

> So from my POV putting the numbers in
> the margins would be a step backward.  It will probably also create
> major havoc for the few packages that do display in the margins,

Not any more havoc than linum or nlinum which many people install and 
use, and are apparently fine with them taking up one of the margins.

> because Emacs facilities for layout of text and other stuff there are
> exceedingly limited.

But of course, it would be ideal if you could also introduce a facility 
that would allow sharing of margins (and hopefully also fringes) between 
different modes.

> Not sure what you mean by "chromeless" here, but if I understand you
> correctly, Martin's work is already on master.

I think so, yes.

>> 1. The first visual line containing the popup has the line number at its
>> beginning. And as such, the popup line is shifted to the right.
> 
> That's the "BOL at non-zero column" issue, right?

Yes, but also the lack of ability to disable the line numbering on a 
line-by-line basis (which could be one solution for this problem).

> The lack of numbers is a "feature", I think: only "physical" lines in
> buffer text are counted, newlines in overlay and display strings do
> not count.

It is indeed a feature from your standpoint, but it's a bug from the 
point of view of the popup's rendering.

Imagine of the line numbers were displayed using line-prefix (I'm not 
really suggesting that, but...). Then the popup could pick them up and 
include in the overlay's `display' property text. The user wouldn't see 
any difference.

> Can you artificially offset the beginning of the overlay to account
> for the line numbers, and see if that alone solves the problem of the
> first lines, and doesn't cause problems in the subsequent lines?

Offset the beginning by how much? By

(- (car (posn-col-row (posn-at-point (point))))
    (car (posn-col-row (posn-at-point (beginning-of-visual-line)))))

? Let's call the value of this expression X.

Let's try a thought experiment.

Suppose X is non-zero because of line numbering. Then adding an offset 
of X to the position of the first popup line should fix it.

What if X is non-zero because of line-prefix, though? If it's because of 
line-prefix variable, okay, we don't support it.

What if X is non-zero because of the line-prefix text property? We'd get 
an "array out of bounds" error somewhere, because the first popup line 
is currently positioned correctly.

So when should we add an X offset to the first popup line's position?

Suppose X is non-nil because of line numbering and the line-prefix text 
property both? Org uses the line-prefix text property in certain 
configurations, so it's a real possibility.

One idea would be to first create an overlay on the first visual line 
which overrides the line-prefix property to zero, *then* measure

(car (posn-col-row (posn-at-point (beginning-of-visual-line))))

, then delete the said overlay and proceed. Doesn't that sound like a 
mess already?