fix: Detect and handle nested Vim instances in Tmux

[pwnalone]

Bug Description

Split navigation stops working after opening/exiting Neovim from within a Neovim terminal window when using the Tmux integrations.

Minimal Config

Neovim

local lazypath = vim.fn.stdpath("data") .. "/lazy/lazy.nvim"
if not vim.loop.fs_stat(lazypath) then
  -- stylua: ignore
  vim.fn.system({ "git", "clone", "--filter=blob:none", "https://github.com/folke/lazy.nvim.git", "--branch=stable", lazypath })
end
vim.opt.rtp:prepend(lazypath)

require("lazy").setup({
  {
    "mrjones2014/smart-splits.nvim",
    event = "VeryLazy",
    config = function()
      local map = function(mode, lhs, rhs, opts)
        local base = { noremap = true, silent = true }
        vim.keymap.set(mode, lhs, rhs, vim.tbl_extend("force", base, opts or {}))
      end

      local ss = require("smart-splits")

      ss.setup()

      map({ "n", "i", "t" }, "<M-h>", ss.move_cursor_left)
      map({ "n", "i", "t" }, "<M-j>", ss.move_cursor_down)
      map({ "n", "i", "t" }, "<M-k>", ss.move_cursor_up)
      map({ "n", "i", "t" }, "<M-l>", ss.move_cursor_right)
    end,
  },
})

Tmux

set -g prefix C-a

bind -n 'M-h' if -F "#{@pane-is-vim}" 'send M-h' 'selectp -L'
bind -n 'M-j' if -F "#{@pane-is-vim}" 'send M-j' 'selectp -D'
bind -n 'M-k' if -F "#{@pane-is-vim}" 'send M-k' 'selectp -U'
bind -n 'M-l' if -F "#{@pane-is-vim}" 'send M-l' 'selectp -R'
bind -n 'M-;' if -F "#{@pane-is-vim}" 'send M-;' 'lastp'

bind -T copy-mode-vi 'M-h' selectp -L
bind -T copy-mode-vi 'M-j' selectp -D
bind -T copy-mode-vi 'M-k' selectp -U
bind -T copy-mode-vi 'M-l' selectp -R
bind -T copy-mode-vi 'M-;' lastp

bind '\' splitw -v -c '#{pane_current_path}'
bind '|' splitw -h -c '#{pane_current_path}'
unbind '"'
unbind '%'

Steps To Reproduce

1. Open Tmux.
2. Open Neovim.
3. Open a Neovim terminal window.
4. Open Neovim from within the Neovim terminal window.
5. Exit the nested Neovim editor.
6. Exit the terminal window.
7. Attempt to navigate between Neovim splits.

Fix Description

This PR fixes the issue by checking if the @pane-is-vim Tmux option is already set to 1 on initialization. If it is, then we set a flag to prevent unsetting the option on exit.

[pwnalone]

No problem! I only ran into this issue because I use Neovim as my man-pager (e.g. export MANPAGER='nvim +Man!') and I sometimes open up man-docs in Neovim terminals. In other words, running nested Neovim instances isn't something I would normally want to do. :)