bug#61529: 30.0.50; tree-sitter: weird off-by-one error but only in css-ts-mode(?) with `treesit-node-at'

[Mickey Petersen <mickey@masteringemacs.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Mickey Petersen <mickey@masteringemacs.org>
>> Date: Wed, 15 Feb 2023 08:25:53 +0000
>>
>>
>> With point at '2', then I'd expect `treesit-node-at' to yield that node. But it does not:
>>
>> (cons (point) (treesit-node-at (point)))
>>
>> => (34 . #<treesit-node "(" in 34-35>)
>
> The value of point is the number of the character which _follows_
> point, yes?  So when the cursor is on '2', point is actually between
> '(' and '2'.  Right?  What does this mean in terms of the node that
> should be returned by tree-sitter?

Correct, point is between '(' and '2'. So 34-35 means it occupies
position 34-35 or [34,35). So point is outside the scope of the '('
single-char anonymous node.

Or at least it should be: the problem is that it *is* inside it in
this one weird instance and, near as I can find, only in this mode,
and then only in this place, it isn't. I suspect `treesit-node-at' has
a bug.

Consider:

    a {
      background: linear-gradient(210deg, rgba(|255,82,41,1) 0%, rgba(251,165,85,1) 54%, rgba(163,73,73,1) 100%);
    }

Note the new position of point in rgba. `treesit-node-at` with `(point)` now correctly returns

    #<treesit-node integer_value in 48-51>

Move point back one position:

    a {
      background: linear-gradient(210deg, rgba|(255,82,41,1) 0%, rgba(251,165,85,1) 54%, rgba(163,73,73,1) 100%);
    }

And now:

  (treesit-node-at (point)) => #<treesit-node "(" in 47-48>

In start contrast to the original example.