widget: constrain drag offset to [0, 1]

Once the user begins dragging, the cursor can move outside the clip
area (or even the window on at least X11), leading to events with
positions that are either negative, or larger than the clip area.

Negative values outright break the delta tracking and cause the
scrollbar to misbehave. Positive values "only" break the invariant of
Scrollbar.ScrollDistance that the returned value is in the range [-1, 1].

Signed-off-by: Dominik Honnef <dominik@honnef.co>

widget/list.go

@@ -74,6 +74,12 @@ func (s *Scrollbar) Layout(gtx layout.Context, axis layout.Axis, viewportStart,
 			continue
 		}
 		dragOffset := axis.FConvert(event.Position).X
+		// The user can drag outside of the constraints, or even the window. Limit dragging to within the scrollbar.
+		if dragOffset < 0 {
+			dragOffset = 0
+		} else if dragOffset > trackHeight {
+			dragOffset = trackHeight
+		}
 		normalizedDragOffset := dragOffset / trackHeight
 
 		if !s.dragging {