Weird characters sent to ssh terminal when Alt+Tab between apps

Question

I ssh to my AWS EC2 machine with Amazon Linux from local Ubuntu with GNOME Terminal. If open vim in the ssh session and then Alt+Tab from terminal to another app and then Alt+Tab back, annoying messages appear: E349: No identifier under cursor or Press ENTER or type command to continue. After some digging I've figured out the reason: weird characters are sent to ssh terminal when I Alt+Tab:

[ec2-user@ip-172-31-43-121 ~]$ cat
^[[O^[[I^[[O^[[I^[[O^[[I^[[O^[[I^[[O^[[I

^[[O^[[I per single Alt+Tab. What do these characters mean? Why are they sent and how to prevent their occurrence?

EDIT: Fixed by restarting GNOME Terminal.

Answer 1

That's the terminal sending xterm's focus-in/focus-out escapes. vim's set the terminal into a mode that lets it know when the cursor leaves the window (which it does when alt-tabbing).

You would prevent that by changing the mode used in vim for handling the mouse. Checking xterm's documentation, vim would have to send a particular escape sequence with 1004 parameter (to tell xterm to send those escapes), but I do not see that in vim's source-code. Some other terminal may be doing this as a side-effect of the mouse-movement mode 1003 (which vim does send):

    if (xterm_mouse_vers > 0)
    {
        if (on) // enable mouse events, use mouse tracking if available
            out_str_nf((char_u *)
               (xterm_mouse_vers > 1
            ? (
#ifdef FEAT_BEVAL_TERM
                bevalterm_ison
                   ? IF_EB("\033[?1003h", ESC_STR "[?1003h") :
#endif
                  IF_EB("\033[?1002h", ESC_STR "[?1002h"))
            : IF_EB("\033[?1000h", ESC_STR "[?1000h")));
        else    // disable mouse events, could probably always send the same
            out_str_nf((char_u *)
                   (xterm_mouse_vers > 1
                ? IF_EB("\033[?1002l", ESC_STR "[?1002l")
                : IF_EB("\033[?1000l", ESC_STR "[?1000l")));
        mouse_ison = on;
    }

You probably should consult the user manual of the terminal you are using, to see what the expected behavior would be with vim.