How to keep cursor vertically aligned to center even at the end of buffer?

Question

I would like to keep my cursor centered vertically as I type away in insert mode even when I reach the end of the buffer I am editing (at which point I would like the virtual lines starting with ~ to populate the screen, or something similar). From this question, I understand that it may be impossible to do this at the top of the file, but since I almost always edit at the bottom, I was wondering if it is possible to maintain this behavior at the bottom of the file. Specifically, this is the behavior I would like to have:

1. As I type or scroll in insert or normal mode at the middle of a long file, the cursor should remain vertically centered.
2. As I reach the bottom, I would like the cursor to remain vertically centered and the screen be populated with virtual lines.
3. If I type at the top, I don't mind the cursor not being vertically centered.

Here is what I have tried:

1. set scrolloff = 999. This works well in the middle of the file, but does not work at the bottom.
2. Remapping scroll keys to append the zz command. This works well in normal mode even at the end of the file, but does not work in insert mode.
3. Running zz in an autocmd in insert mode. Specifically, I tried both of the following:

au InsertCharPre * norm zz
au TextChangedI,TextChangedP * norm zz

Both work, but both have the one weird effect. At the end of a line, whenever I type something, the cursor is positioned before the inserted character. That is, representing cursor with |,

End of line|        <- I type the character '.' here
End of line|.       <- The cursor is now positioned before the '.'

Note that this happens only at the end of a line. At the middle of a line, things seem to work fine. This seems to be a quirk of norm, and maybe somehow running different commands in the autocmd based on weather the cursor is at the end of the line will work, but I feel like there should be a shorter way to achieve what I want.

Any help is much appreciated.

Answer 1

You can fix the cursor position at the end of the line with a little extra Vimscript:

augroup KeepCentered
  autocmd!
  autocmd CursorMoved * normal! zz
  autocmd TextChangedI * call InsertRecenter()
augroup END

function InsertRecenter() abort
  let at_end = getcursorcharpos()[2] > len(getline('.'))
  normal! zz

  " Fix position of cursor at end of line
  if at_end
    let cursor_pos = getcursorcharpos()
    let cursor_pos[2] = cursor_pos[2] + 1
    call setcursorcharpos(cursor_pos[1:])
  endif
endfunction

This is essentially the "running different commands in the autocmd based on whether the cursor is at the end of the line" solution that you rejected in your question, but it feels cleaner to me than my old answer, which involved mapping keys in insert mode:

Old (accepted) answer

Normal/visual mode

For normal/visual mode, rather than remapping scroll commands, I would use an autocommand:

augroup KeepCentered
  autocmd!
  autocmd CursorMoved * normal! zz
augroup END

Insert mode

For insert mode, as you've discovered, simple autocommands won't suffice. Instead, note that there are very few keystrokes that can cause the cursor to change lines when in insert mode: you can remap these to keep the cursor line centred:

inoremap <CR> <C-\><C-O><C-E><CR>
inoremap <BS> <BS><C-O>zz
nnoremap o <C-E>o

(I don't use the arrow keys, but if you use these in insert mode, you can remap <up> and <down> similarly.)