From 33a87ac61c387c39a2b53f1d25dfae5ff9a8986a Mon Sep 17 00:00:00 2001
From: Gordon Klaus <gordon.klaus@gmail.com>
Date: Wed, 3 Jun 2020 11:09:19 +0200
Subject: [PATCH] io/router: deliver scroll events only to the foremost scroll
 handler

Signed-off-by: Gordon Klaus <gordon.klaus@gmail.com>
---
 io/pointer/doc.go         |  2 ++
 io/router/pointer.go      |  5 +++++
 io/router/pointer_test.go | 43 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+)

diff --git a/io/pointer/doc.go b/io/pointer/doc.go
index abc0ef1b..efb0a4b4 100644
--- a/io/pointer/doc.go
+++ b/io/pointer/doc.go
@@ -23,6 +23,8 @@ Leave, or Scroll):
 		Types: pointer.Press | pointer.Move | pointer.Release,
 	}.Add(ops)
 
+Scroll events are only delivered to the foremost scroll handler.
+
 Cancel events are always delivered.
 
 Areas
diff --git a/io/router/pointer.go b/io/router/pointer.go
index 1bd16040..8e27015c 100644
--- a/io/router/pointer.go
+++ b/io/router/pointer.go
@@ -267,6 +267,11 @@ func (q *pointerQueue) deliverEvent(p *pointerInfo, events *handlerEvents, e poi
 		e.Position = h.transform.Invert().Transform(e.Position)
 
 		addPointerEvent(events, k, e, h.types)
+
+		// Only deliver scroll events to the foremost handler.
+		if e.Type == pointer.Scroll && e.Type&h.types != 0 {
+			return
+		}
 	}
 }
 
diff --git a/io/router/pointer_test.go b/io/router/pointer_test.go
index 9f9bee86..9fce1686 100644
--- a/io/router/pointer_test.go
+++ b/io/router/pointer_test.go
@@ -125,6 +125,49 @@ func TestPointerTypes(t *testing.T) {
 	assertEventSequence(t, r.Events(handler), pointer.Cancel, pointer.Press, pointer.Release)
 }
 
+func TestPointerScroll(t *testing.T) {
+	handler1 := new(int)
+	handler2 := new(int)
+	var ops op.Ops
+
+	pointer.Rect(image.Rect(0, 0, 100, 100)).Add(&ops)
+	pointer.InputOp{Tag: handler1, Types: pointer.Scroll}.Add(&ops)
+
+	pointer.Rect(image.Rect(0, 0, 100, 50)).Add(&ops)
+	pointer.InputOp{Tag: handler2, Types: pointer.Scroll}.Add(&ops)
+
+	var r Router
+	r.Frame(&ops)
+	r.Add(
+		// Hit handler 2.
+		pointer.Event{
+			Type: pointer.Scroll,
+			Position: f32.Point{
+				X: 50,
+				Y: 25,
+			},
+		},
+		// Hit handler 1.
+		pointer.Event{
+			Type: pointer.Scroll,
+			Position: f32.Point{
+				X: 50,
+				Y: 75,
+			},
+		},
+		// Hit no handlers.
+		pointer.Event{
+			Type: pointer.Scroll,
+			Position: f32.Point{
+				X: 50,
+				Y: 125,
+			},
+		},
+	)
+	assertEventSequence(t, r.Events(handler1), pointer.Cancel, pointer.Scroll)
+	assertEventSequence(t, r.Events(handler2), pointer.Cancel, pointer.Scroll)
+}
+
 func TestPointerEnterLeave(t *testing.T) {
 	handler1 := new(int)
 	handler2 := new(int)