Stray characters displayed when switching to the shell

[brammool]

Steps to reproduce

This depends on a lot of factors, especially the terminal used and how Vim interacts with it.
Probably the "worst case" is if the terminal supports getting XT termcodes and the 'xtermcodes' option
has not been switched off.

One way to trigger it is to use "CTRL-Z" early in startup, e.g. with:
vim --clean -c ':call feedkeys("\<c-z>", "n")'
Reported by @Shane-XB-Qian on #11705

Expected behaviour

No escape sequences are displayed in the shell

Version of Vim

9.0.1060

Environment

Ubuntu, kitty

Logs and stack traces

No response

[Shane-XB-Qian]

this is not only kitty, but also xfce-term (and with gnu-screen) etc.
but except kitty, most other terminals most those stray chars can be workaround like the similar way at #11728
but just for the reproduce cmd vim --clean -c ':call feedkeys("\<c-z>", "n")',
vs if the cmd is vim --clean -c ':!' -c ':call feedkeys("\<c-z>", "n")' (had -c ':!'),
then there still maybe a ^[[I

it is <focusgained>, perhaps it is v9.0.1065 should take care that also.
// it is related to my vimrc, and test under my xfce-term with gnu-screen.
// this is not a reproduce steps, but just giving a hint about ^[[I perhaps should be cared too.
// anyway, you can figure out something anything later when/if you have time............................

[yjfguhd]

FWIW, this happens in 9.0.948, but not 9.0.447.

[brammool]

Considering the "focus gained" event, I am currently looking into focus reporting.
There are two sides: enabling/disabling focus reporting and handling the escape codes for the events.

There does not appear to be any support in terminfo/termcap for this. The best solution I have come up
with (so far) is to add this to mouse handling. this is the t_XM termcap entry in Vim.
Terminals can add the code in the "XM" entry to make it work. For many recent terminals this value for t_XM
can work: "\033[?1006;1004;1000%?%p1%{1}%=%th%el%;".

The second side is that Vim has to decide to recognize the "focus gained" and "focus lost" events.
Although these are often the same, always recognizing them would mean that for terminals that use
the escape sequence for some key, mapping that key would not work. It is better to only recognize
the events when focus reporting is expected to work.

Besides checking if t_fe is not empty, finding "1004" in t_XM is also a pretty good indication.
I plan to implement this, then we'll see if this works and doesn't cause problems in some environment.