Render UI directly to /dev/tty

See https://github.com/junegunn/fzf/discussions/3792

This allows us to separately capture the standard error from fzf and its
child processes, and there's less chance of user errors of redirecting
the error stream and hiding fzf.

src/tui/light.go

@@ -73,7 +73,7 @@ func (r *LightRenderer) csi(code string) string {
 
 func (r *LightRenderer) flush() {
 	if r.queued.Len() > 0 {
-		fmt.Fprint(os.Stderr, "\x1b[?7l\x1b[?25l"+r.queued.String()+"\x1b[?25h\x1b[?7h")
+		fmt.Fprint(r.ttyout, "\x1b[?7l\x1b[?25l"+r.queued.String()+"\x1b[?25h\x1b[?7h")
 		r.queued.Reset()
 	}
 }
@@ -88,6 +88,7 @@ type LightRenderer struct {
 	prevDownTime  time.Time
 	clicks        [][2]int
 	ttyin         *os.File
+	ttyout        *os.File
 	buffer        []byte
 	origState     *term.State
 	width         int
@@ -131,6 +132,10 @@ func NewLightRenderer(theme *ColorTheme, forceBlack bool, mouse bool, tabstop in
 	if err != nil {
 		return nil, err
 	}
+	out, err := openTtyOut()
+	if err != nil {
+		out = os.Stderr
+	}
 	r := LightRenderer{
 		closed:        util.NewAtomicBool(false),
 		theme:         theme,
@@ -138,6 +143,7 @@ func NewLightRenderer(theme *ColorTheme, forceBlack bool, mouse bool, tabstop in
 		mouse:         mouse,
 		clearOnExit:   clearOnExit,
 		ttyin:         in,
+		ttyout:        out,
 		yoffset:       0,
 		tabstop:       tabstop,
 		fullscreen:    fullscreen,