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

test/test_tmux.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+require 'shellwords'
+require 'tmpdir'
+require_relative 'lib/common'
+
+# Tests for running fzf in a tmux floating pane (--popup on tmux 3.7 or above)
+class TestTmux < TestInteractive
+  def setup
+    super
+    # Cannot rely on the exit status; tmux versions before 3.7 exit
+    # normally with empty output for an unknown command name
+    supported = IO.popen(%w[tmux list-commands new-pane], err: File::NULL, &:read).include?('new-pane')
+    skip('floating panes not supported') unless supported
+  end
+
+  def test_floating_pane
+    tmux.send_keys "seq 100 | #{fzf('--popup center,80% --margin 0')}", :Enter
+    tmux.until { |lines| assert_equal 100, lines.item_count }
+    tmux.send_keys '99'
+    tmux.until { |lines| assert_equal 1, lines.match_count }
+    tmux.send_keys :Enter
+    assert_equal '99', fzf_output
+  end
+
+  def test_floating_pane_killed
+    tmux.send_keys "seq 100 | #{FZF} --popup bottom,50% --margin 0; echo code:$?", :Enter
+    tmux.until { |lines| assert_equal 100, lines.item_count }
+    pane = floating_pane
+    refute_nil pane
+    assert system('tmux', 'kill-pane', '-t', pane)
+    tmux.until { |lines| assert lines.any_include?('code:130') }
+  end
+
+  def test_border_fzf_falls_back_to_popup
+    # display-popup requires an attached client, which the test environment
+    # may not have; intercept it with a tmux shim on PATH
+    dir = Dir.mktmpdir
+    real = `command -v tmux`.chomp
+    shim = File.join(dir, 'tmux')
+    File.write(shim, <<~SH)
+      #!/bin/sh
+      if [ "$1" = display-popup ]; then
+        echo popup-used >&2
+        exit 0
+      fi
+      exec #{real.shellescape} "$@"
+    SH
+    FileUtils.chmod(0o755, shim)
+    tmux.send_keys "seq 100 | PATH=#{dir.shellescape}:$PATH #{FZF} --popup center,border-fzf", :Enter
+    tmux.until { |lines| assert lines.any_include?('popup-used') }
+    refute floating_pane
+  ensure
+    FileUtils.remove_entry(dir) if dir
+  end
+
+  private
+
+  def floating_pane
+    format = "\#{pane_id} \#{pane_floating_flag}"
+    lines = IO.popen(['tmux', 'list-panes', '-t', tmux.win, '-F', format]) { |io| io.readlines(chomp: true) }
+    lines.filter_map { |line| line.split.first if line.end_with?(' 1') }.first
+  end
+end