layout: truncate negative List.Position.First positions to 0

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

layout/list.go

@@ -65,10 +65,10 @@ type Position struct {
 	BeforeEnd bool
 	// First is the index of the first visible child.
 	First int
-	// Offset is the distance in pixels from the top edge to the child at index
+	// Offset is the distance in pixels from the leading edge to the child at index
 	// First.
 	Offset int
-	// OffsetLast is the signed distance in pixels from the bottom edge to the
+	// OffsetLast is the signed distance in pixels from the trailing edge to the
 	// bottom edge of the child at index First+Count.
 	OffsetLast int
 	// Count is the number of visible children.
@@ -95,6 +95,10 @@ func (l *List) init(gtx Context, len int) {
 	l.children = l.children[:0]
 	l.len = len
 	l.update(gtx)
+	if l.Position.First < 0 {
+		l.Position.Offset = 0
+		l.Position.First = 0
+	}
 	if l.scrollToEnd() || l.Position.First > len {
 		l.Position.Offset = 0
 		l.Position.First = len

layout/list_test.go

@@ -13,6 +13,25 @@ import (
 	"gioui.org/op"
 )
 
+func TestListPositionExtremes(t *testing.T) {
+	var l List
+	gtx := Context{
+		Ops:         new(op.Ops),
+		Constraints: Exact(image.Pt(20, 10)),
+	}
+	const n = 3
+	layout := func(_ Context, idx int) Dimensions {
+		if idx < 0 || idx >= n {
+			t.Errorf("list index %d out of bounds [0;%d]", idx, n-1)
+		}
+		return Dimensions{}
+	}
+	l.Position.First = -1
+	l.Layout(gtx, n, layout)
+	l.Position.First = n + 1
+	l.Layout(gtx, n, layout)
+}
+
 func TestEmptyList(t *testing.T) {
 	var l List
 	gtx := Context{