Why can't I set <Home> or t_kh in my vimrc file?

Question

I'm having some problems remapping my home key; it works when I run Vim from tmux but not when I run it from a regular xterm without tmux. It looks like they're sending different escape codes and Vim only picks up on tmux's one for some reason.

I tried mucking about with a lot of TERM values, to no avail. So I figured I'd just hard-code this escape code in my vimrc and move on:

set nocompatible

" `^[` is the escape character.
set <Home>=^[OH
set t_kh=^[OH

echo &t_kh

noremap <Home> :echoerr "home!"<CR>

Starting with $ vim -u vimrc Vim outputs ^[OH, which is expected. But after I've started Vim the setting seems to be not set:

:set t_kh?
E846: Key code not set: t_kh?
:verbose set t_kh?
E846: Key code not set: t_kh?

And the remapped home key doesn't work. Manually running :set <Home> or :set t_kh fixes the problem; I can now use my custom <Home> mapping.

Why aren't these options taking effect? How can I make them take effect?

I have no plugins and am using Vim 8.0.586 on Arch Linux.

Answer 1

I found this in :help xterm-function-keys:

An xterm can send function keys F1 to F4 in two modes: vt100 compatible or
not.  Because Vim may not know what the xterm is sending, both types of keys
are recognized.  The same happens for the <Home> and <End> keys.
                        normal                  vt100
        <F1>    t_k1    <Esc>[11~       <xF1>   <Esc>OP     <xF1>-xterm
        <F2>    t_k2    <Esc>[12~       <xF2>   <Esc>OQ     <xF2>-xterm
        <F3>    t_k3    <Esc>[13~       <xF3>   <Esc>OR     <xF3>-xterm
        <F4>    t_k4    <Esc>[14~       <xF4>   <Esc>OS     <xF4>-xterm
        <Home>  t_kh    <Esc>[7~        <xHome> <Esc>OH     <xHome>-xterm
        <End>   t_@7    <Esc>[4~        <xEnd>  <Esc>OF     <xEnd>-xterm

When Vim starts, <xF1> is mapped to <F1>, <xF2> to <F2> etc.  This means that
by default both codes do the same thing.  If you make a mapping for <xF2>,
because your terminal does have two keys, the default mapping is overwritten,
thus you can use the <F2> and <xF2> keys for something different.

Setting both <Home> and <xHome> seems to work well! Because we only want this to take effect inside xterm I added a &term check (tmux is screen-256color):

set nocompatible

if &term == "xterm-256color"
    " `^[` is the escape character.
    set <Home>=^[OH
    set <xHome>=^[OH
endif

noremap <Home> :echoerr "home!"<CR>

I'd be lying if I'd say that I fully comprehend all of the above, but it seems to work well without ill effects in my testing.