io/router,app: scroll a bit when reaching the end in a focus direction

List was recently changed to include an extra child at each end, to
automatically scroll when reaching the end of a focus direction. However,
if List includes unfocusable children that strategy may fail. This change
adds another fallback where app.Window will scroll a constant amount in
the focus direction, to reveal more children.

For https://github.com/tailscale/tailscale/issues/4278.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/window.go

@@ -69,6 +69,8 @@ type Window struct {
 	delayedDraw  *time.Timer
 	// viewport is the latest frame size with insets applied.
 	viewport image.Rectangle
+	// metric is the metric from the most recent frame.
+	metric unit.Metric
 
 	queue       queue
 	cursor      pointer.Cursor
@@ -504,10 +506,28 @@ func (c *callbacks) MoveFocus(dir router.FocusDirection) {
 }
 
 func (w *Window) moveFocus(dir router.FocusDirection, d driver) {
-	w.queue.q.MoveFocus(dir)
+	if w.queue.q.MoveFocus(dir) {
+		w.queue.q.RevealFocus(w.viewport)
+	} else {
+		var v image.Point
+		switch dir {
+		case router.FocusRight:
+			v = image.Pt(+1, 0)
+		case router.FocusLeft:
+			v = image.Pt(-1, 0)
+		case router.FocusDown:
+			v = image.Pt(0, +1)
+		case router.FocusUp:
+			v = image.Pt(0, -1)
+		default:
+			return
+		}
+		const scrollABit = 50
+		dist := v.Mul(w.metric.Px(unit.Dp(scrollABit)))
+		w.queue.q.ScrollFocus(dist)
+	}
 	w.setNextFrame(time.Time{})
 	w.updateAnimation(d)
-	w.queue.q.ScrollFocus(w.viewport)
 }
 
 func (c *callbacks) ClickFocus() {
@@ -748,6 +768,7 @@ func (w *Window) processEvent(d driver, e event.Event) {
 			// No drawing if not visible.
 			break
 		}
+		w.metric = e2.Metric
 		var frameStart time.Time
 		if w.queue.q.Profiling() {
 			frameStart = time.Now()
@@ -771,7 +792,7 @@ func (w *Window) processEvent(d driver, e event.Event) {
 		}
 		// Scroll to focus if viewport is shrinking in any dimension.
 		if old, new := w.viewport.Size(), viewport.Size(); new.X < old.X || new.Y < old.Y {
-			w.queue.q.ScrollFocus(viewport)
+			w.queue.q.RevealFocus(viewport)
 		}
 		w.viewport = viewport
 		size := e2.Size // save the initial window size as the decorations will change it.

io/router/key.go

@@ -169,7 +169,8 @@ func (q *keyQueue) updateFocusLayout() {
 	}
 }
 
-func (q *keyQueue) MoveFocus(dir FocusDirection, events *handlerEvents) {
+// MoveFocus attempts to move the focus in the direction of dir, returning true if it succeeds.
+func (q *keyQueue) MoveFocus(dir FocusDirection, events *handlerEvents) bool {
 	order := 0
 	if q.focus != nil {
 		order = q.handlers[q.focus].dirOrder
@@ -194,6 +195,7 @@ func (q *keyQueue) MoveFocus(dir FocusDirection, events *handlerEvents) {
 		}
 		order = (order + len(q.order)) % len(q.order)
 		q.setFocus(q.order[order], events)
+		return true
 	case FocusRight, FocusLeft:
 		next := order
 		if q.focus != nil {
@@ -206,6 +208,7 @@ func (q *keyQueue) MoveFocus(dir FocusDirection, events *handlerEvents) {
 			newFocus := q.dirOrder[next]
 			if newFocus.row == focus.row {
 				q.setFocus(newFocus.tag, events)
+				return true
 			}
 		}
 	case FocusUp, FocusDown:
@@ -242,8 +245,10 @@ func (q *keyQueue) MoveFocus(dir FocusDirection, events *handlerEvents) {
 		}
 		if closest != nil {
 			q.setFocus(closest, events)
+			return true
 		}
 	}
+	return false
 }
 
 func (q *keyQueue) Push(e event.Event, events *handlerEvents) {

io/router/key_test.go

@@ -285,7 +285,7 @@ func TestFocusScroll(t *testing.T) {
 	r.Frame(ops)
 
 	r.MoveFocus(FocusLeft)
-	r.ScrollFocus(image.Rect(0, 0, 15, 40))
+	r.RevealFocus(image.Rect(0, 0, 15, 40))
 	evts := r.Events(h)
 	assertScrollEvent(t, evts[len(evts)-1], f32.Pt(5, -10))
 }

io/router/router.go

@@ -149,13 +149,13 @@ func (q *Router) Queue(events ...event.Event) bool {
 	return q.handlers.HadEvents()
 }
 
-func (q *Router) MoveFocus(dir FocusDirection) {
-	q.key.queue.MoveFocus(dir, &q.handlers)
+func (q *Router) MoveFocus(dir FocusDirection) bool {
+	return q.key.queue.MoveFocus(dir, &q.handlers)
 }
 
-// ScrollFocus scrolls the current focus (if any) into viewport
+// RevealFocus scrolls the current focus (if any) into viewport
 // if there are scrollable parent handlers.
-func (q *Router) ScrollFocus(viewport image.Rectangle) {
+func (q *Router) RevealFocus(viewport image.Rectangle) {
 	focus := q.key.queue.focus
 	if focus == nil {
 		return
@@ -175,11 +175,20 @@ func (q *Router) ScrollFocus(viewport image.Rectangle) {
 	if s.Y == 0 {
 		s.Y = bottomright.Y
 	}
+	q.ScrollFocus(s)
+}
+
+// ScrollFocus scrolls the focused widget, if any, by dist.
+func (q *Router) ScrollFocus(dist image.Point) {
+	focus := q.key.queue.focus
+	if focus == nil {
+		return
+	}
 	area := q.key.queue.AreaFor(focus)
 	q.pointer.queue.Deliver(area, pointer.Event{
 		Type:   pointer.Scroll,
 		Source: pointer.Touch,
-		Scroll: fpt(s),
+		Scroll: fpt(dist),
 	}, &q.handlers)
 }