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Date: Wed, 10 Apr 2019 16:03:23 -0400 (EDT)
From: Thomas Dickey <dickey@his.com>
To: Shaddy Baddah <lithium-cygwin@shaddybaddah.name>
Cc: cygwin <cygwin@cygwin.com>
Message-ID: <1919747550.7813512.1554926603758.JavaMail.zimbra@his.com>
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Subject: Re: *cause of* screen writing over restored buffer on detach/exit
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----- Original Message -----
| From: "Shaddy Baddah" <lithium-cygwin@shaddybaddah.name>
| To: "cygwin" <cygwin@cygwin.com>
| Sent: Tuesday, April 9, 2019 11:18:19 PM
| Subject: Re: *cause of* screen writing over restored buffer on detach/exit

| On 9/4/19 4:07 pm, Shaddy Baddah wrote:
|> 
|> This helped with screen when using Putty to a Cygwin ssh session. For
|> some reason, it isn't helping for running screen locally in a mintty
|> session. And it's not mintty either, because I can ssh to a Debian
|> stretch server within mintty and I can use its screen without this
|> issue happening. Back to the drawing board for me.
| 
| I understand the cause of the issue now, by capturing and comparing the
| escape characters used to control the terminal by screen on Cygwin and
| Debian.
| 
| It is not so much the detach/exit is the issue, although the escape
| sequence for Cygwin includes a couple of extra xterm OSC Set Text
| Parameters -> Change Icon Name and Window Title outputs. I misdiagnosed
| this somehow. I can suppress the extra utmp error one of these OSC
| outputs, but it didn't/doesn't really make a difference.
| 
| The issue is in the escape sequences sent to the terminal by Cygwin
| screen to switch to new windows buffer, as compared to Debian.
| 
| Debian uses xterm sequence DECSET / ESC[?1049h in the switch to the new
| window. Cygwin uses the two sequences ESC7 / Save Cursor (DECSC) and
| ESC[?47l / DEC Private Mode Reset (DECRST) -> Use Normal Screen Buffer.
| 
| 
| The reason seems to be that the Debian screen package packages a custom
| /etc/screenrc that does not include this explicit term capability:
| 
| 
| #
| # Do not use xterms alternate window buffer.
| # This one would not add lines to the scrollback buffer.
| termcap xterm|xterms|xs ti=\E7\E[?47l
| terminfo xterm|xterms|xs ti=\E7\E[?47l
| 
| 
| 
| If I comment these out, my screen issue is resolved.
| 
| I'm not suggesting this is a problem with Cygwin screen... it is using
| the upstream settings. In fact, I am not confident to say where the
| fault lies. Perhaps screen is right to use these sequences, but the
| xterms used (putty and mintty) aren't doing the right thing?

yes - that's one of a half-dozen cases where PuTTY has never matched xterm's behavior.

-- 
Thomas E. Dickey <dickey@invisible-island.net>
http://invisible-island.net
ftp://ftp.invisible-island.net

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