Rendering errors in tmux split panes

[sw00]

Environment

Windows build number: Microsoft Windows [Version 10.0.19042.388]
Windows Terminal version (if applicable): 1.0.1811.0
tmux 3.0a
Alacritty 0.4.2

Steps to reproduce

Run tmux in Windows terminal WSL2 (Ubuntu), after a fresh boot into Windows.
Create some horizontal split panes in tmux (works)
Create some vertical split panes in tmux
Run some commands with output (in some panes)

Oddly, the symptom disappears if I close and start Windows terminal again and re-attach to tmux.

Expected behavior

Text output is clear and readable and output in their respective panes.

Actual behavior

The text output from different processes seems to bleed across panes.
This occurs with a vanilla/empty tmux config too.

My TERM is set to xterm-256color outside tmux and tmux-256color in tmux.

This behaviour also seems to happen in Alacritty (0.4.2), which in my understanding shares the same backend (ConPTY?) as Windows Terminal.

[sschlesier]

I have observed the same.

It worked well with the first official release and then broke in the first official update.

I installed a preview release in which it was working, but also recently broke with an update.

I was about to try and test all the recent releases to see where it does/does not work. I noticed an odd thing. If I launch WSL2 and get a tmux session running in an old working version of the official terminal, and then attach a broken version of the preview terminal to the same tmux session the display will remain correct in both terminals. However if I close both terminals and wsl -t to terminate the wsl session, and then launch wsl and tmux again with a broken terminal it will not render correctly. That surprised me and hopefully is a helpful clue.

For now I've run out of time to try and track this down.

[sschlesier]

Oh, I missed the close and start again comment. Maybe that is the factor that has made me think certain versions were fixed/broken and explains the render keeps working with both the preview and release versions.

[kianryan]

Observing identical problems with:
Windows Terminal Version: 1.1.2021.0
Tmux 3.0a
WSL 2 / Ubuntu 20.04
TERM=xterm-256color (outside)
TERM=tmux-256color (inside)

[zadjii-msft]

First - very sorry that this got lost in the triage queue.

Second - does this only repro in tmux 3.0a? Maybe they're using something new in 3.0 that we don't support quite yet? I'm sitting here on 18.04 with tmux 2.6 and I just can not repro this. Lemme pull a 20.04 and see if I can't get this to happen with an updated tmux.

[zadjii-msft]

Huh. I could not get this to repro with a fresh ubuntu 20.04 and tmux 3.0a, UNTIL I tried installing fish:

I'm not sure if it'll happen again, but I could repro this at least once, so there's something.

EDIT: tmux-256color is certainly a weird termcap:

[zadjii-msft]

Alright other findings here

• Once a Windows Terminal pane is corrupted by this, it will stay corrupted. With the pane that's busted for me, every time I try killing all my tmux sessions and opening a new one in that WT pane, it's got the same visual corruption as OP.
• The corruption seems to be caused by tmux or conpty causing the mouse to go back to the 1st column instead of where the pane separator should be. maybe there's a bug with calculating where the tab stops should be?
  • tmux is redrawing the contents of the bottom right pane correctly, but when I force a redraw of the entire window, then tmux will draw the corrupted rows starting from column 0, instead of the separator.
    

[zadjii-msft]

Alright last update:

This has definitely got something to do with line wrapping. My gut thinks that this is related to #6901/#7028/#7253, but I can't be sure, since I can't get a solid repro right now.

Take a look at this output copied from a broken tmux and a working one. They should both have the same contents in their panes:

Note that the first one has all copied as one single wrapped line.

I've also got the attached tracking of me opening tmux, making a [|] split, and running htop in it:
gh-6987-trace.txt

There's nothing terribly out of the ordinary there. I split the pane around L958, and start htop around 2215.

I'm gonna toss this on V2, and link it up with #5800. If anyone's got a way to more consistently repro this from a fresh WT window, that'd be much appreciated. Thanks all!