Use floating pane instead of popup on tmux 3.7 or above (#4850) (#4852)

Unlike a popup, a floating pane is not modal; you can switch to other
panes and windows while fzf is running, move and resize the pane with
the mouse, zoom it to fullscreen, and use copy-mode in it.

- Floating pane always has a native border, so 'border-native' is
  implied; give new 'border-fzf' option to fall back to a popup where
  fzf draws its own border
- Popup is also used on tmux versions below 3.7, or when the window is
  too small to fit a floating pane
- new-pane does not block until the command finishes and does not
  propagate the exit status; block on a wait-for channel signaled by
  the pane and pass the exit status through a temporary file
- Watchdog process signals the channel when the pane is closed
  abnormally (e.g. kill-pane)
- Kill the pane when the proxy process is interrupted, like a popup
  dying with its client
- Unzoom the window before creating the floating pane; doing so over a
  zoomed window crashes the tmux server on 3.7b, and newer versions of
  tmux unzoom the window anyway
- Floating pane size excludes the border and the position is that of
  the content area; treat the requested size as the total footprint
  including the border for consistency with popups
- Close the pane on exit even when remain-on-exit is on
- Pre-create the exit status file with O_EXCL to prevent tampering on
  a shared TMPDIR

man/man1/fzf.1

@@ -417,11 +417,19 @@ layout options so that the specified number of items are visible in the list
 section (default: \fB10+\fR).
 Ignored when \fB\-\-height\fR is not specified or set as an absolute value.
 .TP
-.BI "\-\-popup" "[=[center|top|bottom|left|right][,SIZE[%]][,SIZE[%]][,border-native]]"
-Start fzf in a tmux popup or in a Zellij floating pane (default
-\fBcenter,50%\fR). Requires tmux 3.3+ or Zellij 0.44+. This option is ignored if you
+.BI "\-\-popup" "[=[center|top|bottom|left|right][,SIZE[%]][,SIZE[%]][,border-native|border-fzf]]"
+Start fzf in a tmux or Zellij floating pane (default \fBcenter,50%\fR).
+Requires tmux 3.3+ or Zellij 0.44+. This option is ignored if you
 are not running fzf inside tmux or Zellij. \fB\-\-tmux\fR is an alias for this option.
 
+On tmux 3.7 or above, the floating pane is not modal; you can switch to
+other panes and windows while fzf is running, move and resize the pane with
+the mouse, zoom it to fullscreen, and use copy\-mode in it. The pane always
+has a native border, which is what makes it movable and resizable, so
+\fBborder\-native\fR is implied. On tmux versions below 3.7, or when
+\fBborder\-fzf\fR is given, a modal tmux popup is used instead and fzf draws
+its own border.
+
 e.g.
   \fB# Popup in the center with 70% width and height
   fzf \-\-popup 70%