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Subject: Re: Screen clearing in CMD without "Legacy Console Mode"
To: cygwin@cygwin.com
References: <YIwTSMfJu9uMwrKn@kevinlocke.name>
 <20210430235358.2ae6daa41e3f0b6cb0daa173@nifty.ne.jp>
From: Brian Inglis <Brian.Inglis@SystematicSw.ab.ca>
Organization: Systematic Software
Message-ID: <b8a7facb-0ef4-317a-9c5a-f7c47941ddf6@SystematicSw.ab.ca>
Date: Fri, 30 Apr 2021 10:40:33 -0600
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On 2021-04-30 08:53, Takashi Yano via Cygwin wrote:
> On Fri, 30 Apr 2021 08:25:12 -0600
> Kevin Locke wrote:
>> Hi All,
>>
>> 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.
>>
>> The difference appears to result from enabling
>> SetConsoleMode(ENABLE_VIRTUAL_TERMINAL_PROCESSING) and
>> SetConsoleMode(ENABLE_VIRTUAL_TERMINAL_INPUT)[^3] to behave like
>> xterm-256color when supported[^4] (they are not supported in "Legacy
>> Console Mode").
>>
>> I'm not too familiar with TTY/PTY handling, much less Cygwin on top of
>> CMD.  It's not clear to me if the alternate screen buffer behaves
>> differently in CMD than xterm, whether Cygwin has any responsibility, or
>> if the issue is in how CMD handles ANSI escape sequences.  Johannes
>> Schindelin suggested it may make sense not to enable virtual terminal
>> processing when TERM=cygwin is set.[^5]  Thoughts?
>>
>> Any advice or assistance for how to address this issue would be
>> appreciated.
>>
>> Thanks,
>> Kevin
>>
>> [^1]: https://github.com/git-for-windows/git/issues/3177
>> [^2]: https://docs.microsoft.com/windows/console/legacymode
>> [^3]: https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/fhandler_console.cc;h=ebe89fc9f8fd9fcb8102ee9884b8b91f04e88fb1;hb=5eb232ede72e394e280d1a23131267c7002d2c52#l464
>> [^4]: https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/fhandler_console.cc;h=ebe89fc9f8fd9fcb8102ee9884b8b91f04e88fb1;hb=5eb232ede72e394e280d1a23131267c7002d2c52#l1327
>> [^5]: https://github.com/git-for-windows/git/issues/3177#issuecomment-828494565
> 
> 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.

Was this a change made to Cygwin and from which version?

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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