Re: [bug-ncurses] Emacs difficulties in linux console with ncurses-6.3 caused by kcbt=\E^I.

[Dr. Werner Fink]

On 2022/05/05 20:01:02 -0400, Thomas Dickey wrote:
> On Thu, May 05, 2022 at 06:18:00PM +0000, Alan Mackenzie wrote:
> > Hello, ncurses!
> > 
> > I'm writing as a member of the Emacs development team.
> > 
> > In the recent change from ncurses-6.2 to ncurses-6.3, the following
> > change was make in the linux console terminfo:
> > 
> > --- Infocmp-linux-6.2   2022-05-04 20:16:01.609557894 +0000
> > +++ infocmp-linux-6.3   2022-05-04 20:09:02.046581014 +0000
> > @@ -14,7 +14,7 @@
> >         home=\E[H, hpa=\E[%i%p1%dG, ht=^I, hts=\EH, ich=\E[%p1%d@,
> >         ich1=\E[@, il=\E[%p1%dL, il1=\E[L, ind=\n,
> >         
> > initc=\E]P%p1%x%p2%{255}%*%{1000}%/%02x%p3%{255}%*%{1000}%/%02x%p4%{255}%*%{1000}%/%02x,
> > -       kb2=\E[G, kbs=^?, kcbt=\E[Z, kcub1=\E[D, kcud1=\E[B,
> > +       kb2=\E[G, kbs=^?, kcbt=\E^I, kcub1=\E[D, kcud1=\E[B,
> >         kcuf1=\E[C, kcuu1=\E[A, kdch1=\E[3~, kend=\E[4~, kf1=\E[[A,
> >         kf10=\E[21~, kf11=\E[23~, kf12=\E[24~, kf13=\E[25~,
> >         kf14=\E[26~, kf15=\E[28~, kf16=\E[29~, kf17=\E[31~,
> > 
> > We now have kcbt=\E^I.
> 
> A bug report got me to verify this, and
> shift-tab on the console appeared to send that sequence,
> so I documented it in the terminal description.
> 
> Tracing changes to default keyboard configuration in Linux isn't
> as "simple" as control-sequences, but see below (it's "kbd").
> 
> I don't see any that send \E[Z.  Having the terminal description
> list a key definition that no one uses isn't very useful.
> (I'll allow for some keyboard differences -- who has a "clear" key? --
> but shift-tab has been used for a long time).
> 
> Just to check:
>       Debian, Fedora, Mageia, OpenSUSE do this (send \E^I)
>       Arch, Slackware don't do this (I get just ^I)
> 
> I have a few others that I could check, but (CentOS and Scientific Linux)
> those are either obsolete or derived from the ones that I listed.
> 
> I'm aware that there are (down in the 1% range) still other Linux-based
> systems, but generally speaking few/none of those have contributed
> in this area, so I don't have a machine to verify bug reports.
> 
> fwiw, I noticed some comment in the usual source of misinformation
> recently stating that \E[Z originated with Linux in 1995.
> That was incorrect (the terminal database shows this for several
> AT&T entries and an Ann Arbor Ambassador entry - roughly ten
> years earlier).
>  
> > In Emacs on the Linux console, our use of terminfo now has the effect of
> > replacing the M-tab keysequence with backtab.  This make several
> > important key-bindings unusable.  (The backtab binding is also used, but
> > is perhaps a little less important.)
> 
> Perhaps that original kcbt was changed to allow the \E^I to work
> with Emacs.  If that's the case, then it should be something that
> I can document in the terminal description.
> 
> All that I have so far is the commit message, which gives no useful
> information regarding the intent of the change:
> 
>       commit 0baa7f071e79bb700b62b1f8507630387cbc4bbb
>       Author: Alexey Gladkov <legion@altlinux.org>
>       Date:   Tue May 8 22:55:07 2007 +0400
> 
>           Apply patchkbd-1.12-Meta-Tab.diff from SUSE
>           
>           Convert Shift Tab to Meta_Tab
>           
>           Signed-off-by: Alexey Gladkov <legion@altlinux.org>
> 
> and this equally non-informative mailing list thread:
> 
> https://lists.opensuse.org/archives/list/commit@lists.opensuse.org/message/P3DQHCZB7IWQ7F6VZD3LGXHVOIN6OFE6/
> 
> (which for example doesn't mention Emacs).


This was a bug here from 2002

https://bugzilla.opensuse.org/show_bug.cgi?id=31512

interestingly I had never a bug report against ncurses here

> 
> > Also, this would seem to be a difficult problem for a normal user to
> > diagnose, and difficult also to fix.
> > 
> > May I ask why this change was made to the linux terminal?  Is there any
> > possibiliy it might be reverted?
> 
> It was a bug report:
> 
> # 2021-09-04
> #     + modify linux3.0 entry to reflect default mapping of shift-tab by
> #       kbd 1.14 (report by Jan Engelhardt) -TD
>  
> > Is there perhaps some strategy we could use in Emacs (C code) which would
> > work around this problem with using ugly ad-hoc code?
> 
> I suppose you could tell Emacs to ignore kcbt (termcap kB) for Linux.
> 
> non-Emacs users probably would like to use the key.
> 

Werner

-- 
  "Having a smoking section in a restaurant is like having
          a peeing section in a swimming pool." -- Edward Burr

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