io/router: don't panic if AppendSemantics is called without a frame

Fixes: https://todo.sr.ht/~eliasnaur/gio/328
Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/router/pointer.go

@@ -321,12 +321,18 @@ func (c *pointerCollector) offerOp(op transfer.OfferOp, events *handlerEvents) {
 	h.data = op.Data
 }
 
-func (c *pointerCollector) reset(q *pointerQueue) {
+func (c *pointerCollector) reset() {
-	q.reset()
+	c.q.reset()
 	c.resetState()
 	c.nodeStack = c.nodeStack[:0]
-	c.q = q
+	c.ensureRoot()
-	// Add implicit root area for semantic descriptions to hang onto.
+}
+
+// Ensure implicit root area for semantic descriptions to hang onto.
+func (c *pointerCollector) ensureRoot() {
+	if len(c.q.areas) > 0 {
+		return
+	}
 	c.pushArea(areaRect, f32.Rect(-1e6, -1e6, 1e6, 1e6))
 	// Make it semantic to ensure a single semantic root.
 	c.q.areas[0].semantic.valid = true

io/router/router.go

@@ -183,13 +183,16 @@ func (q *Router) SemanticAt(pos f32.Point) (SemanticID, bool) {
 // AppendSemantics appends the semantic tree to nodes, and returns the result.
 // The root node is the first added.
 func (q *Router) AppendSemantics(nodes []SemanticNode) []SemanticNode {
+	q.pointer.collector.q = &q.pointer.queue
+	q.pointer.collector.ensureRoot()
 	return q.pointer.queue.AppendSemantics(nodes)
 }
 
 func (q *Router) collect() {
 	q.transStack = q.transStack[:0]
 	pc := &q.pointer.collector
-	pc.reset(&q.pointer.queue)
+	pc.q = &q.pointer.queue
+	pc.reset()
 	kc := &q.key.collector
 	*kc = keyCollector{q: &q.key.queue}
 	q.key.queue.Reset()

io/router/semantic_test.go

@@ -15,6 +15,14 @@ import (
 	"gioui.org/op/clip"
 )
 
+func TestEmptySemantics(t *testing.T) {
+	var r Router
+	tree := r.AppendSemantics(nil)
+	if len(tree) != 1 {
+		t.Errorf("expected 1 semantic node for empty tree, got %d", len(tree))
+	}
+}
+
 func TestSemanticTree(t *testing.T) {
 	var (
 		ops op.Ops