Soft wrap scrolling issue

Description of the problem or steps to reproduce

Note: Terminal is 80x24

1. Open a file with long lines (Example: https://pastebin.com/FTUyuaUp which is from Tor Project's newsletter)
2. Turn softwrap on with set softwrap on
3. Scroll down to line ~15
4. Open help with ctrl+g
5. Ctrl+w back to the file
6. Scroll up
7. You will observe that sometimes scrolling up moves the cursor out of the window (most often after long multiline lines)

Specifications

Version: 2.0.4-dev.15
Commit hash: 55e9759
Compiled on May 14, 2020
OS: Artix Linux
Terminal: gnome-terminal, 80x24

Related to #1598 and #632. As frabjous said, all these issues are consequences of the core implementation of softwrapping. Not an easy change, I imagine, but one of the biggest things Micro could use.

[flber]

I would love to see this fixed, specifically the problem as worded in #632, as it is my major frustration with micro.

[dmaluka]

I'm working on fixing these softwrap scrolling issues (as worded in #632). It's pretty much working for me already. Hopefully I'll prepare a PR soon.

The core idea of my implementation is simple: whenever doing any operations on the buffer view (scrolling, centering, relocating etc), work not with simple line numbers but with (Line, Row) pairs, where Line is a logical line number and Row is a visible line (row) number within a logical line. The logic basically remains the same, but simple arithmetic operations on line numbers are replaced with corresponding operations on (Line, Row) pairs.

If I finish it and it gets merged, it will be possible to implement further improvements on top of it: moving cursor up and down within a wrapped line (#1598) and word wrapping (#264, #1644).

[SolarLune]

I believe I am also seeing this issue - currently, if you enable softwrap while the buffer is scrolled away from the first column, it doesn't scroll back to the start, regardless of where the cursor is.

[dmaluka]

@SolarLune it seems you are talking about the horizontal scrolling issue #645. It is not fixed in #1981 but I hope to fix it soon too.

[SolarLune]

Ah, my mistake. Thanks for the direction; I'll subscribe to that issue.

[dmaluka]

I believe this can be closed, since #1981 is merged.