Allow optionally disabling the use of TABs for TTY cursor movement

* src/term.c (syms_of_term) <tty-cursor-movement-use-TAB>: New var.
<tty-cursor-movement-use-TAB-BS>: Doc fix.
* src/cm.c (calccost): Use it to disable use of TABs for cursor
motion on text terminals.

* etc/NEWS: Announce the new variable.

1 files changed, 12 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index a5806a99e31..e764545c508 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -37,6 +37,18 @@ exit status to 256 if sending input to that process returned EPIPE.
 Now when this happens, Emacs closes the file descriptor to write to the
 child process, but allows it to continue execution as normal.
 
+---
+** New variable 'tty-cursor-movement-use-TAB'.
+If this is set to the nil value, Emacs will not use TABs to optimize
+cursor motion on text-mode terminals.  This is for the rare cases where
+the hardware tabs of the terminal were set to a non-default value by the
+'tabs' command or similar, or if using TABs for cursor movement has any
+other undesired effects.  The default is t, which preserves past
+behavior.
+
+If this variable is nil, 'tty-cursor-movement-use-TAB-BS' has no effect,
+and Emacs will never use TABs for any cursor-movement sequences.
+
 
 * Editing Changes in Emacs 32.1