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

src/proxy.go

@@ -26,14 +26,14 @@ func escapeSingleQuote(str string) string {
 func popupArgStr(args []string, opts *Options) (string, string) {
 	fzf, rest := args[0], args[1:]
 	args = []string{"--bind=ctrl-z:ignore"}
-	if !opts.Tmux.border && (opts.BorderShape == tui.BorderUndefined || opts.BorderShape == tui.BorderLine) {
+	if opts.Tmux.border != tmuxBorderNative && (opts.BorderShape == tui.BorderUndefined || opts.BorderShape == tui.BorderLine) {
 		if tui.DefaultBorderShape == tui.BorderRounded {
 			rest = append(rest, "--border=rounded")
 		} else {
 			rest = append(rest, "--border=sharp")
 		}
 	}
-	if opts.Tmux.border && opts.Margin == defaultMargin() {
+	if opts.Tmux.border == tmuxBorderNative && opts.Margin == defaultMargin() {
 		args = append(args, "--margin=0,1")
 	}
 	argStr := escapeSingleQuote(fzf)