[9fans] Bug: Acme: Scrolling is not deterministic for long lines

[kalterdev@gmail.com]

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Expected behavior

When I scroll with button 3, the line next to the cursor always goes
to the beginning of the visible area. Similarly, If the line is long
and broken (wordwrapped) by Acme, clicking on the line's second part
pushes it to the beginning of the visible area.

Clicking button 1 on the same place must, and in most cases does, pull
the line back to its original place. If the line is long and broken by
Acme, though, button 1 must guarantee that the visible part of the
line goes back.

Unexpected behavior

If the first visible line is long and broken by Acme, button 1 does
not pull it's visible part to the cursor's location.

Steps to reproduce the problem

The problem typically occurs when Acme breaks long lines. Find a long
line, make Acme break it, and scroll with buttons 3 and 1 on the
line's first or second part.

Another weird behavior is exhibited by the first visible line broken
by Acme. Try to scroll it's second part with buttons 3 and 1. On
button 1, the whole line goes to its original place except it's first
character.

See demo for visual demonstration of the two cases.

https://i.imgur.com/S1dyDlU.mp4

Sometimes even the beginning of the long line does not return. I
couldn't identify the exact cause, though, but there's a demo.

https://i.imgur.com/kKNXKC1.mp4

plan9port version

a2567fcac9851e5cc965a236679f568b0e79cff2

OS version

macOS 14.5 (23F79)

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