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

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- **Smooth scrolling on huge terminals:** while an arrow key is held, the poster waits until the
+  selection rests (120 ms). Every poster rewrites all of its cells, which on a terminal zoomed far
+  out was more output per step than the terminal could draw, so moving through a list lagged.
 - **Posters in macOS Terminal:** Terminal.app draws the half and quarter blocks from the font, and
   their halves do not meet at the middle of the cell, so posters looked like rows of shifted dots.
   `auto` now draws them there as plain colored cells (new `ui.cover_style` value `cells`): coarser,

internal/ui/detail.go

@@ -3,6 +3,7 @@ package ui
 import (
 	"image"
 	"strings"
+	"time"
 
 	"github.com/gdamore/tcell/v2"
 	"github.com/rivo/tview"
@@ -14,6 +15,12 @@ const (
 	minCoverRows = 6 // a smaller poster is not worth showing
 	minCoverCols = 8
 	minTextRows  = 6 // the poster never takes these rows from the text
+
+	// posterSettle: when the selection moves again within this time (a held
+	// arrow key), the poster waits until the selection rests that long. Every
+	// poster rewrites all of its cells, which on a huge terminal (zoomed far
+	// out) is more output than the terminal can draw at key-repeat speed.
+	posterSettle = 120 * time.Millisecond
 )
 
 // detailPane shows the details of one title as one flowing text with the
@@ -27,9 +34,11 @@ type detailPane struct {
 	cover       *coverView
 
 	detail   detailText
-	lastRef  coverRef // the poster of the current detail, to fetch it again after a style change
-	want     string   // key of the poster that should be shown
-	hasCover bool     // a poster (or its loading note) is part of the content
+	lastRef  coverRef    // the poster of the current detail, to fetch it again after a style change
+	want     string      // key of the poster that should be shown
+	hasCover bool        // a poster (or its loading note) is part of the content
+	lastShow time.Time   // when the shown title last changed
+	settle   *time.Timer // shows the poster once the selection rests
 
 	// What the text was last wrapped for.
 	laidOut struct {
@@ -61,7 +70,34 @@ func (d *detailPane) Show(text detailText, ref coverRef) {
 		d.cover.setImage(nil)
 		return
 	}
+	now := time.Now()
+	moving := ref.key != d.want && now.Sub(d.lastShow) < posterSettle
+	if ref.key != d.want {
+		d.lastShow = now
+	}
 	d.want, d.hasCover = ref.key, true
+	if d.settle != nil {
+		d.settle.Stop()
+	}
+	if moving {
+		// Keep the poster's place (so the text does not reflow) but leave it
+		// empty until the selection stops.
+		d.cover.setNote("")
+		want := ref.key
+		d.settle = time.AfterFunc(posterSettle, func() {
+			d.app.post(func() {
+				if d.want == want {
+					d.loadPoster(ref)
+				}
+			})
+		})
+		return
+	}
+	d.loadPoster(ref)
+}
+
+// loadPoster shows the poster of ref, fetching it first if needed.
+func (d *detailPane) loadPoster(ref coverRef) {
 	d.cover.setNote("loading cover…")
 	want := ref.key
 	d.app.loadCover(ref, func(img *image.RGBA, err error) {

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")
+	}
+}