io/input/pointer: ignore grab commands for tags that don't have the pointer

Without this fix, two gestures that both issue GrabCmd on the same frame
will cancel each other. With the fix, the first will win the grab, and
the other ignored.

References: https://todo.sr.ht/~eliasnaur/gio/647
Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/input/pointer.go

@@ -257,6 +257,17 @@ func (q *pointerQueue) grab(state pointerState, req pointer.GrabCmd) (pointerSta
 		if !p.pressed || p.id != req.ID {
 			continue
 		}
+		// Verify that the grabber is among the handlers.
+		found := false
+		for _, tag := range p.handlers {
+			if tag == req.Tag {
+				found = true
+				break
+			}
+		}
+		if !found {
+			continue
+		}
 		// Drop other handlers that lost their grab.
 		for i := len(p.handlers) - 1; i >= 0; i-- {
 			if tag := p.handlers[i]; tag != req.Tag {

io/input/pointer_test.go

@@ -97,6 +97,39 @@ func TestPointerDragNegative(t *testing.T) {
 	assertEventPointerTypeSequence(t, events(&r, -1, f), pointer.Enter, pointer.Press, pointer.Leave, pointer.Drag)
 }
 
+func TestIgnoredGrab(t *testing.T) {
+	handler1 := new(int)
+	handler2 := new(int)
+	var ops op.Ops
+
+	filter := func(t event.Tag) event.Filter {
+		return pointer.Filter{Target: t, Kinds: pointer.Press | pointer.Release | pointer.Cancel}
+	}
+
+	event.Op(&ops, handler1)
+	event.Op(&ops, handler2)
+	var r Router
+	assertEventPointerTypeSequence(t, events(&r, -1, filter(handler1)), pointer.Cancel)
+	assertEventPointerTypeSequence(t, events(&r, -1, filter(handler2)), pointer.Cancel)
+	r.Frame(&ops)
+	r.Queue(
+		pointer.Event{
+			Kind:     pointer.Press,
+			Position: f32.Pt(50, 50),
+		},
+		pointer.Event{
+			Kind:     pointer.Release,
+			Position: f32.Pt(50, 50),
+		},
+	)
+	assertEventPointerTypeSequence(t, events(&r, 1, filter(handler1)), pointer.Press)
+	assertEventPointerTypeSequence(t, events(&r, 1, filter(handler2)), pointer.Press)
+	r.Source().Execute(pointer.GrabCmd{Tag: handler1})
+	r.Source().Execute(pointer.GrabCmd{Tag: handler2})
+	assertEventPointerTypeSequence(t, events(&r, 1, filter(handler1)), pointer.Release)
+	assertEventPointerTypeSequence(t, events(&r, 1, filter(handler2)), pointer.Cancel)
+}
+
 func TestPointerGrab(t *testing.T) {
 	handler1 := new(int)
 	handler2 := new(int)