widget: move scrollbar indicator if dragging starts outside of it

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

widget/list.go

@@ -87,7 +87,22 @@ func (s *Scrollbar) Layout(gtx layout.Context, axis layout.Axis, viewportStart,
 		if !s.dragging {
 			s.dragging = true
 			s.oldDragPos = normalizedDragOffset
+
+			if normalizedDragOffset < viewportStart || normalizedDragOffset > viewportEnd {
+				// The user started dragging somewhere on the track that isn't covered by the indicator. Consider this a
+				// click in addition to a drag and jump to the clicked point.
+				//
+				// TODO(dh): this isn't perfect. We only get the pointer.Drag event once the user has actually dragged,
+				// which means that if the user presses the mouse button and neither releases it nor drags it, nothing
+				// will happen.
+				pos := axis.Convert(image.Point{
+					X: int(event.Position.X),
+					Y: int(event.Position.Y),
+				})
+				normalizedPos := float32(pos.X) / trackHeight
+				s.delta += normalizedPos - viewportStart
 			}
+		} else {
 			s.delta += normalizedDragOffset - s.oldDragPos
 
 			if viewportStart+s.delta < 0 {
@@ -103,6 +118,7 @@ func (s *Scrollbar) Layout(gtx layout.Context, axis layout.Axis, viewportStart,
 			}
 			s.oldDragPos = normalizedDragOffset
 		}
+	}
 
 	// Process events from the indicator so that hover is
 	// detected properly.