mintty update broke scrollback buffer support in left/right margins mode with default margins

[pa-jberanek]

So, I was previously using wsltty 1.9.6, connecting to a CentOS 7 server where I had tmux installed, and configured to disable the alternate screen buffer, so I could still use what I consider to be a valuable fast local scrollback (with Shift-PageUp/Down or scrollbar), and all was good.

Recently I updated to wsltty 3.0.2, and this has stopped working. It's annoyed me enough that I've gone back to using PuTTY, but I do still prefer the wsltty solution generally.

Tmux is configured with the following to disable the alternate screen buffer:

set -ga terminal-overrides ",xterm*:smcup@:rmcup@,putty*:smcup@:rmcup@,screen*:smcup@:rmcup@"

[Biswa96]

Are you using Windows Subsystem for Linux (WSL) to connect to that server? If not then use normal cygwin or msys2 installation.

[pa-jberanek]

Yes, I run WSL inside mintty with wsltty, and from there connect to Linux servers.

[Biswa96]

What is the Windows 10 version?

[pa-jberanek]

I'm on 1903 (18362)

[Biswa96]

Hand over to @mintty.

[mintty]

configured to disable the alternate screen buffer

Configured mintty (SuppressSGR or NoAltScreen) or only via tmux?

Tmux is configured with the following to disable the alternate screen buffer:

Tmux on WSL or on the CentOS server?

Please try NoAltScreen=true in %APPDATA%\wsltty\config.
Did you change anything (e.g. tmux upgrade) on the server side about the same time?
Can you activate a screen log, to check whether the switching sequences were/are being sent?
Can you describe the test case more precisely...?

[pa-jberanek]

I've not configured mintty for anything alternate buffer related.

The tmux is on the CentOS server, and I've not changed the version there.

I can try and capture a log of me reproducing it. All I do is:

On wsltty session, SSH to the Linux server
Start fresh tmux session
Do 'ls /usr/bin' to put some data in my scrollback
Attempt Shift-PageUp or scrollbar moving

All I see is the session from before I started tmux, and nothing of the overflowed 'ls' output.

[pa-jberanek]

Setting NoAltScreen=true didn't change the behaviour. Attached is a capture from 'script' on the WSL instance.
typescript.gz

[mintty]

Ah, I've got a suspicion. Can you make the same typescript with the previous wsltty version to compare?
Is there actually a tool to replay that in real-time?
Anyway, mintty 3.0.1 added support for VT420 left/right margin mode, and that mode is set in the typescript. When scrolling in margin mode, lines scrolled out are not put into the scrollback buffer (because normally, not whole lines but only partial lines would be scrolled).

Now we could investigate why your tmux uses margin mode now and did not before (or to check whether it tried in the first place, thus my request above).

Also, I'm actually wondering: if you want to use local scrollback, why do you use tmux at all?

[mintty]

The log contains the sequence \e[?69h\e[1;36r\e[s which sets left/right margins to their default.
This makes no sense, it's better to run with left/right margins mode disabled.
Therefore I consider this a bug of the application, tmux.
On the other hand, xterm handles this special case and maintains the scrollback buffer, so mintty can change that too.

[pa-jberanek]

To answer your question why I run tmux at all: I run it so that I can maintain 10 persistent consoles on a remote server, so I'm not reliant on connections being continuously open on my workstation.

It's still much faster/more natural to use a local scrollback buffer, though presumably I could fix the "naturalness" by changing tmux's keybindings for scrolling.

[pa-jberanek]

So, downgrading to wsltty 1.9.6/mintty 2.9.6 does return the previous behaviour, typescript attached.

typescript-mintty-2.9.6.gz

[mintty]

In the "older" log, left/right margin mode is in fact not activated. This is weird if you really changed nothing else. Does the application perhaps check the mintty version?
Anyway, mintty will be changed to catch this case in the next version.

[mintty]

Released 3.0.3.

[pa-jberanek]

Thanks, verified with mintty 3.0.5.