Why does custom map of <c-w><space> cause mapping delay in terminal mode?

Question

I'm using <c-w><space> to open current terminal buffer in a new tab:

tnoremap <c-w><space> <c-w>:tab split<cr>

This map cause <c-w><c-w> trigger mapping delay in terminal mode, why would that happen? Are there any relation between <space> (0x20) and <c-w> (0x17) ?

The <c-w><space> works fine, it doesn't trigger mapping delay.

I have created an rc file with this single command and tested it with vim -Nu rc, the result didn't change.

FWIW, I also use this map in normal mode:

nnoremap <c-w><space> :tab split<cr>

It works fine, it doesn't affect <c-w><c-w> int normal mode.

I'm using vim8.1-2300, ubuntu 18.04.4, zsh 5.4.2

Answer 1

For some reason, C-w is not written in the typeahead immediately (not printed in the 'showcmd' area).

It seems that Vim waits for &timeoutlen ms to determine whether the key must be remapped or executed with its default meaning (start a C-W command). I guess that the key assigned to 'termwinkey' is handled differently in Terminal-Job mode compared to C-w in normal mode.

You could somewhat get around the timeout by installing this mapping:

tno <c-w><c-w> <c-w><c-w>

This works under the assumption that you didn't reset 'termwinkey', and as a result Vim uses <C-w> to start a C-W command. This wouldn't work if you've reset the option to, let's say, <C-s>, and you press <C-s><C-w> to focus another window.

For something more reliable, you could try this:

augroup termwinkey_no_timeout
    au!
    au TerminalWinOpen * let b:_twk = &l:twk == '' ? '<c-w>' : &l:twk
      \ | exe printf('tno <buffer><nowait> %s<c-w> %s<c-w>', b:_twk , b:_twk)
      \ | unlet! b:_twk
augroup END