The behavior of bufname is unclear to me

Question

I use NERDTree and try to apply the recipe that ensures no one replaces its buffer.

I do the following:

• Start vim
• Execute :NERDTreeFocus
• Open some file
• Execute :q for file buffer
• (At this point there is one window with NERDTree only)
• Open some other file

There I have NERDTree and former file buffers swapped.

While trying to track down the issue I found the following. Suppose we have this autocmd:

autocmd BufEnter * echomsg "Alternate: " . bufnr('#') . " " . bufname('#') | echomsg "Current: " . bufnr('%') . " " . bufname('%')

Then I try:

1. Start vim
2. :vs
3. :e 123
4. <C-W> j
5. <C-W> l

After step 3 I see the following in messages:

"123" [New]
Alternate: 1
Current: 2 123

After step 4:

Alternate: -1
Current: 1

After step 5:

Alternate: 1
Current: 2 123

My question is: why do I have Alternate: -1 after step 4 instead of Alternate: 2 123?

Answer 1

vim stores an alternate file for each window.

1. when vim starts up a new empty bufnr 1 is initialized
2. split, now bufnr 1 is in both windows
3. edit, since bufnr 1 is in both windows, vim must create a new bufnr 2 for the file 123
4. switching to the initial window, where there was no alternate alternate file (# = -1)
5. switching back to second window, where alternate file was bufnr = 1