[BUG] BufReadFifo hook parameter reports overlapping spans

[Screwtapello]

Version of Kakoune

2024.05.18-132-gdaa7e37e

Reproducer

(echo a; sleep 1; echo b; sleep 1; echo c) |
    kak -n -e 'hook global BufReadFifo .* %{ echo -debug %val{hook_param} }; buffer *debug*'

Outcome

The *debug* buffer reports:

1.1,1.2
1.2,2.2
2.2,3.2

Expectations

The description of BufReadFifo in :doc hooks says:

The hook param contains the range of text that was just inserted, in a format compatible with the select command.

I understand this to mean that the range covers exactly the characters just read from the FIFO. Since a character can't be read more than once from a FIFO, I would expect each character to be covered by exactly one range. However, the character at 1.2 appears in both the first and second events, while the character at 2.2 appears in both the second and third events. Instead, I would expect output like:

1.1,1.2
2.1,2.2
3.1,3.2

Additional information

No response

[krobelus]

Thanks, I had started working on this but didn't have a test, only a suspicion that something is wrong.
Namely the way read_fifo() juggles around the final \n to keep scrolling or not (depending on edit -scroll).
Given that this is so tricky, I hope we can get rid of this newline and simply implement autoscrolling via hooks or options.

[mawww]

Its going to be hard to have something as nice as just doing ge on a fifo buffer to have scrolling regardless of how we got into that buffer.

[krobelus]

true; we should maybe also make gj work, which currently only works if the last BufReadFifo had a trailing newline.

a related thing I broke is scrolling in stdin buffers. That should be solvable