Posters wait while the selection moves quickly

On a terminal zoomed far out a poster is tens of thousands of cells, and
redrawing it on every step of a held arrow key was more output than the
terminal could draw (about 100 KB per step at 600x180), so the list
lagged. A selection that changes again within 120 ms keeps the poster's
place empty and shows the poster once it rests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/ui/detail_test.go

@@ -1,8 +1,11 @@
 package ui
 
 import (
+	"context"
+	"image"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/gdamore/tcell/v2"
 	"github.com/rivo/tview"
@@ -107,3 +110,30 @@ func TestDetailPaneFlowsAroundPoster(t *testing.T) {
 		t.Error("a short pane must drop the poster")
 	}
 }
+
+func TestPosterWaitsWhileTheSelectionMoves(t *testing.T) {
+	a := newTestApp(t)
+	img := image.NewRGBA(image.Rect(0, 0, 20, 30))
+	a.coverCache.imgs["one"], a.coverCache.imgs["two"] = img, img
+	ref := func(key string) coverRef {
+		return coverRef{key: key, fetch: func(context.Context) ([]byte, error) { return nil, nil }}
+	}
+	d := newDetailPane(a)
+
+	d.Show(detailText{head: "One"}, ref("one"))
+	if d.cover.img != img {
+		t.Fatal("the first selection should show its poster at once")
+	}
+	d.Show(detailText{head: "Two"}, ref("two"))
+	if d.cover.img != nil || !d.hasCover {
+		t.Errorf("a quick move should keep the poster's place empty: img %v, hasCover %v", d.cover.img != nil, d.hasCover)
+	}
+	d.settle.Stop() // the test has no event loop to deliver it
+
+	// The same title again (a refresh) is not a move.
+	d.lastShow = time.Now().Add(-time.Second)
+	d.Show(detailText{head: "Two"}, ref("two"))
+	if d.cover.img != img {
+		t.Error("a selection at rest should show its poster")
+	}
+}