[vim] Open fzf in a tmux or Zellij floating pane by default (#4905)

The window fzf was started from stays visible, and can be used while fzf
runs, which a popup inside Vim cannot offer.

Only where the pane is not modal, so tmux 3.7 or above, or Zellij 0.44 or
above with fzf 0.71.0 or above. A tmux popup from 3.3 to 3.6 cannot be left,
and below 3.3 fzf goes through the fzf-tmux script, whose options are
spelled differently. Both keep the window inside Vim. An explicit popup
layout is unaffected and still works from 3.3.

fzf checks tmux before Zellij, so tmux wins when both are set.

g:fzf_layout still wins, so anyone who set it sees no change.

README-VIM.md

@@ -1,6 +1,26 @@
 FZF Vim integration
 ===================
 
+<!-- vim-markdown-toc GFM -->
+
+* [Installation](#installation)
+* [Summary](#summary)
+* [`:FZF[!]`](#fzf)
+    * [Configuration](#configuration)
+        * [Examples](#examples)
+            * [Explanation of `g:fzf_colors`](#explanation-of-gfzf_colors)
+* [`fzf#run`](#fzfrun)
+* [`fzf#wrap`](#fzfwrap)
+    * [Global options supported by `fzf#wrap`](#global-options-supported-by-fzfwrap)
+* [Tips](#tips)
+    * [fzf inside terminal buffer](#fzf-inside-terminal-buffer)
+    * [Starting fzf in a Vim popup window](#starting-fzf-in-a-vim-popup-window)
+    * [Starting fzf in a tmux/Zellij popup window](#starting-fzf-in-a-tmuxzellij-popup-window)
+    * [Hide statusline](#hide-statusline)
+* [License](#license)
+
+<!-- vim-markdown-toc -->
+
 Installation
 ------------
 
@@ -133,19 +153,35 @@ let g:fzf_action = {
   \ 'ctrl-v': 'vsplit' }
 
 " Default fzf layout
-" - Popup window (center of the screen)
-let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
+if exists('$TMUX') || exists('$ZELLIJ')
+  " The Vim plugin will try to open fzf in a tmux or Zellij popup
+  " if possible (requires recent fzf and tmux/zellij) using --popup option,
+  " with the following argument:
+  let g:fzf_layout = { 'popup': '90%,60%' }
+else
+  " If --popup option is not available, it will open in a popup window inside
+  " Vim (center of the screen)
+  let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
+endif
 
-" - Popup window (center of the current window)
+" Here are some more layout examples:
+
+" - Tmux or Zellij popup at the bottom 40%
+let g:fzf_layout = { 'popup': 'bottom,40%' }
+
+" - Tmux or Zellij popup at the top with a different size
+let g:fzf_layout = { 'popup': 'top,90%,40%' }
+
+" - Vim popup window: at the center of the current window (relative)
 let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6, 'relative': v:true } }
 
-" - Popup window (anchored to the bottom of the current window)
+" - Vim popup window: anchored to the bottom of the current window
 let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6, 'relative': v:true, 'yoffset': 1.0 } }
 
-" - down / up / left / right
+" - Vim split window: down / up / left / right
 let g:fzf_layout = { 'down': '40%' }
 
-" - Window using a Vim command
+" - Vim window using a Vim command
 let g:fzf_layout = { 'window': 'enew' }
 let g:fzf_layout = { 'window': '-tabnew' }
 let g:fzf_layout = { 'window': '10new' }
@@ -408,10 +444,10 @@ Tips
 
 ### fzf inside terminal buffer
 
-On the latest versions of Vim and Neovim, fzf will start in a terminal buffer.
-If you find the default ANSI colors to be different, consider configuring the
-colors using `g:terminal_ansi_colors` in regular Vim or `g:terminal_color_x`
-in Neovim.
+When fzf is configured to start in a terminal buffer inside Vim or Neovim, you
+may find the default ANSI colors to be different. In that case, configure the
+colors using `g:terminal_ansi_colors` in regular Vim or `g:terminal_color_x` in
+Neovim.
 
 ```vim
 " Terminal colors for seoul256 color scheme
@@ -442,7 +478,10 @@ else
 endif
 ```
 
-### Starting fzf in a popup window
+### Starting fzf in a Vim popup window
+
+You can configure fzf to start in a Vim popup window by setting the `window` key
+in `g:fzf_layout`.
 
 ```vim
 " Required:
@@ -458,18 +497,32 @@ endif
 let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
 ```
 
-Alternatively, you can make fzf open in a popup window (requires tmux 3.3 or
-above, or Zellij 0.44 or above) by putting `--popup` option value in `popup`
-key. `tmux` is accepted as a synonym, just as `--tmux` is an alias of
-`--popup`.
+### Starting fzf in a tmux/Zellij popup window
+
+fzf can also start in a popup of the multiplexer instead of a window inside
+Vim, by putting a `--popup` option value in the `popup` key. `tmux` is
+accepted as a synonym, just as `--tmux` is an alias of `--popup`.
+
+The layout works on tmux 3.3 or above, or on Zellij 0.44 or above with fzf
+0.71.0 or above. It is the default on tmux 3.7 or above with fzf 0.74.0 or
+above, and on Zellij, where the pane is not modal: Vim keeps the window fzf
+was started from visible, and you can switch to it while fzf is open. Below
+those versions tmux gives a popup that cannot be left, so a window inside Vim
+is the default there. On tmux, an explicit `--border` style also gives a modal
+popup rather than a floating pane, because the native border of a tmux
+floating pane cannot be removed. Drop `--border` to keep the floating pane and
+its native border. Zellij keeps the floating pane either way, and hides its
+native border when fzf draws one. Set `g:fzf_layout` yourself to choose either
+one.
 
 ```vim
-" See `--popup` option in `man fzf` for available options
-" [center|top|bottom|left|right][,SIZE[%]][,SIZE[%]]
 if exists('$TMUX') || exists('$ZELLIJ')
+  " See `--popup` option in `man fzf` for available options
+  " [center|top|bottom|left|right][,SIZE[%]][,SIZE[%]]
   let g:fzf_layout = { 'popup': '90%,70%' }
 else
-  let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
+  " Configure the Vim popup window in case not on the multiplexer
+  let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.7 } }
 endif
 ```
 
@@ -477,7 +530,8 @@ endif
 
 When fzf starts in a terminal buffer, the file type of the buffer is set to
 `fzf`. So you can set up `FileType fzf` autocmd to customize the settings of
-the window.
+the window. This applies to the layouts that open inside Vim, not to the tmux
+or Zellij pane the default uses, which is not a buffer.
 
 For example, if you open fzf on the bottom on the screen (e.g. `{'down':
 '40%'}`), you might want to temporarily disable the statusline for a cleaner