fix(console): clear the spinner line before stopSpinner returns

Stopping only signaled the spinner goroutine, which could sleep up to
80ms before clearing the line — so an error printed right after landed
on the leftover "Starting agent" text. Block the stop function until
the clear escape has been written.

Author: u9g

cmd/lk/console_tui.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 	"strings"
 	"time"
@@ -51,22 +52,30 @@ var spinnerFrames = []string{"⠋", "⠙", "⠹", "⠸", "⠼", "⠴", "⠦", "
 // startSpinner shows a braille spinner on stderr with the given message.
 // Returns a stop function that clears the spinner line.
 func startSpinner(msg string) func() {
+	return startSpinnerTo(os.Stderr, msg)
+}
+
+func startSpinnerTo(w io.Writer, msg string) func() {
 	done := make(chan struct{})
+	cleared := make(chan struct{})
 	go func() {
-		i := 0
-		for {
+		defer close(cleared)
+		for i := 0; ; i++ {
+			fmt.Fprintf(w, "\r  %s %s", spinnerFrames[i%len(spinnerFrames)], msg)
 			select {
 			case <-done:
-				fmt.Fprintf(os.Stderr, "\r\033[K")
+				fmt.Fprintf(w, "\r\033[K")
 				return
-			default:
-				fmt.Fprintf(os.Stderr, "\r  %s %s", spinnerFrames[i%len(spinnerFrames)], msg)
-				i++
-				time.Sleep(80 * time.Millisecond)
+			case <-time.After(80 * time.Millisecond):
 			}
 		}
 	}()
-	return func() { close(done) }
+	// Stopping blocks until the line is cleared, so the caller's next print
+	// (e.g. an error) never lands on the leftover spinner text.
+	return func() {
+		close(done)
+		<-cleared
+	}
 }
 
 type consoleTickMsg struct{}

cmd/lk/console_tui_test.go

@@ -0,0 +1,35 @@
+// Copyright 2026 LiveKit, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestStopSpinner_ClearsLineBeforeReturning(t *testing.T) {
+	// Models the caller printing an error right after stopping the spinner:
+	// by the time stop returns, the spinner line must already be cleared, or
+	// the error lands on the same line as the leftover "⠋ Starting agent".
+	var buf bytes.Buffer
+	stop := startSpinnerTo(&buf, "Starting agent")
+	stop()
+	out := buf.String()
+	require.True(t, strings.HasSuffix(out, "\r\033[K"),
+		"spinner line not cleared when stop returned; output: %q", out)
+}