Screen clearing in CMD without "Legacy Console Mode"

[Johannes Schindelin]

Hi,

On Fri, 30 Apr 2021, Kevin Locke wrote:

> On Fri, 2021-04-30 at 23:53 +0900, Takashi Yano wrote:
> > On Fri, 30 Apr 2021 08:25:12 -0600 Kevin Locke wrote:
> >> I'm investigating an issue in Git for Windows[^1], which also affects
> >> Cygwin.  The issue is that, when using CMD (i.e. Command Prompt) on
> >> Windows 10 1703 or above with "Legacy Console Mode"[^2] disabled, if
> >> TERM=cygwin is set in the environment, the console is not cleared when
> >> vi exits.  To demonstrate, with Cygwin 3.2.0, in CMD with "Legacy
> >> Console Mode" disabled:
> >>
> >> cd C:\cygwin64
> >> set TERM=cygwin
> >> bin\vi etc\bash.bashrc
> >> :q
> >>
> >> After exiting vi, the console window has not been cleared and content
> >> from etc\bash.bashrc remains visible, making further use of the console
> >> difficult until cleared.
> >
> > Why on earth do you want to set TERM=cygwin?
> > If you don't set TERM=cygwin, TERM is automatically set to
> > xterm-256color, in which the issue does not occur.

TERM=cygwin would be correct right until the time when you toggle
`ENABLE_VIRTUAL_TERMINAL_PROCESSING` under our feet.

That is, when the Cygwin process is called, this flag is not set, so
`TERM=xterm-256color` would be incorrect. And then when the Cygwin process
returns, the flag is set, and the calling process should somehow guess
that `TERM` should be set differently.

Oh, and you probably expect the caller to then figure out whether the flag
was _actually_ toggled, just in case we're running on a Windows 10 version
that is too old to understand that flag.

If you expect CMD to be no longer used after the Cygwin process returns,
that might be a valid world view. But that is probably not a very tenable
world view.

If Cygwin is unable to handle `TERM=cygwin` correctly when
`ENABLE_VIRTUAL_TERMINAL_PROCESSING` is toggled, then Cygwin should
definitely, absolutely, with 100% certainty _not_ toggle that flag when
`TERM` is already set to `cygwin`!

Ciao,
Johannes

> Unfortunately, I am not clear on that myself.[^6]  According to Johannes
> Schindelin[^7]:
>
> > We specifically set TERM so that Cygwin (or more correctly, the MSYS2 runtime) uses ANSI sequences...
>
> Hopefully he (or another of the Git for Windows contributors) can add
> more specifics.
>
> Thanks,
> Kevin
>
> [^6]: https://github.com/git-for-windows/git/issues/3177#issuecomment-828507812
> [^7]: https://github.com/git-for-windows/git/issues/3177#issuecomment-826834976
>