bug#36836: Tmux and Focus-In Events

[Gustaf Waldemarson]

Hello,

I noticed a rather strange and somewhat hard to track down bug that's related to
tmux and terminal focus-in events.

Using Emacs 27 and tmux 2.1: If you start tmux with the option `tmux set-option
-g focus-events on` enabled and then inside tmux start an emacsclient, e.g.,
via:

```
emacs -Q --daemon=test && emacsclient --socket-name=test ."
```

Then Emacs starts and unexpectedly inserts the characters "[I]" in the buffer
that is opened. This only seem to happen the first time the client is started.

A quick bisection on the Emacs code-base lead me to this commit:

```
commit 6fdc3fac5658a7ab142c358cddd90f3db5665ef5
Author: Daniel Colascione <dancol@dancol.org>
Date:   Fri Jun 8 22:47:27 2018 -0700
    Support terminal focus notifications

    * lisp/frame.el (handle-focus-in,handle-focus-out): Make event
    argument optional.
    (blink-cursor-check): Make sure that the current frame is a
    window-system frame before restarting the blink timer. TTY frames
    can get focus, but don't need a blink timer because the terminal
    will do the blinking.

    * lisp/term/xterm.el
    (xterm-handle-focus-in,xterm-handle-focus-out): New functions.
    (xterm-rxvt-function-map): Recognize focus notification sequences.
    (xterm--init-focus-tracking): New function.
    (terminal-init-xterm): Call it.
```

Apparently it seems like "\e[I" is the focus-in event, which is translated to
"[I]" by `electric-pair-mode`.

I'm not entirely sure where the bug is in this case however. Is it up to Emacs
to handle focus-in events from the terminal or is that something tmux itself
should handle in this case? Or is there some setting somewhere that should be
toggled to prevent this kind of thing from happening?

I originally observed this behavior in Konsole, but the same thing happen in
xterm itself as well, and possibly other terminal emulators.



In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2019-07-29 built on E105888
Repository revision: a9593253e90aa58134833078ae1fc5505ae9f58c
Repository branch: master
System Description: Ubuntu 16.04.6 LTS

Recent messages:
Mark set
Quit
Mark set
Detected language: English [9 times]
Auto-saving...done
Saving file /home/guswal01/test.md...
Wrote /home/guswal01/test.md
Mark set [2 times]
(No changes need to be saved) [2 times]
Making completion list... [3 times]

Configured using:
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