emacs-28 1d012e0a62: Linux console: don't translate ESC TAB to `backtab' in input-decode-map.

[Alan Mackenzie]

branch: emacs-28
commit 1d012e0a621f9cf99048f57130144275b9025d1c
Author: Alan Mackenzie <acm@muc.de>
Commit: Alan Mackenzie <acm@muc.de>

    Linux console: don't translate ESC TAB to `backtab' in input-decode-map.
    
    This translation happened after the terminfo entry for <shift>TAB in the 
linux
    section was changed to kcbt=\E^I in ncurses version 6.3.
    
    * lisp/term/linux.el (terminal-init-linux): Add a define-key form to remove
    the entry for "\e\t" from input-decode-map.
    
    * etc/PROBLEMS: Add a new section under "character terminals" about S-TAB
    wrongly doing the same thing as M-TAB, giving tips about amending the Linux
    keyboard layout.
---
 etc/PROBLEMS       | 27 +++++++++++++++++++++++++++
 lisp/term/linux.el |  4 ++++
 2 files changed, 31 insertions(+)

diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index 2a26dfaec4..8a260c3177 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -1736,6 +1736,33 @@ this, you can remove the X resource or put this in your 
init file:
 
   (xterm-remove-modify-other-keys)
 
+** The shift TAB key combination works as meta TAB on a Linux console.
+
+This happens because on your keyboard layout, S-TAB produces the same
+keycodes as typing ESC TAB individually.  The best way to solve this
+is to modify your keyboard layout to produce different codes, and tell
+Emacs what these new codes mean.
+
+The current keyboard layout will probably be a .map.gz file somewhere
+under /usr/share/keymaps.  Identify this file, possibly from a system
+initialization file such as /etc/conf.d/keymaps.  Run gunzip on it to
+decompress it, and amend the entries for keycode 15 to look something
+like this:
+
+keycode  15 = Tab
+        alt     keycode  15 = Meta_Tab
+        shift keycode 15 = F219
+string F219 = "\033[4}\011"      # Shift+<tab>
+
+After possibly saving this file under a different name, compress it
+again using gzip.  Amend /etc/conf.d/keyamps, etc., if needed.
+Further details can be found in the man page for loadkeys.
+
+Then add the following line near the start of your site-start.el or
+.emacs or init.el file:
+
+(define-key input-decode-map "\e[4}\t" 'backtab)
+
 ** Emacs spontaneously displays "I-search: " at the bottom of the screen.
 
 This means that Control-S/Control-Q (XON/XOFF) "flow control" is being
diff --git a/lisp/term/linux.el b/lisp/term/linux.el
index 6d43e477ac..ab5a6d8698 100644
--- a/lisp/term/linux.el
+++ b/lisp/term/linux.el
@@ -17,6 +17,10 @@
 
   (ignore-errors (when gpm-mouse-mode (require 't-mouse) (gpm-mouse-enable)))
 
+  ;; Don't translate ESC TAB to backtab as directed
+  ;; by ncurses-6.3.
+  (define-key input-decode-map "\e\t" nil)
+  
   ;; Make Latin-1 input characters work, too.
   ;; Meta will continue to work, because the kernel
   ;; turns that into Escape.