widget: consider size of indicator when limiting scrollbar dragging

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

widget/list.go

@@ -89,6 +89,18 @@ func (s *Scrollbar) Layout(gtx layout.Context, axis layout.Axis, viewportStart,
 			s.oldDragPos = normalizedDragOffset
 		}
 		s.delta += normalizedDragOffset - s.oldDragPos
+
+		if viewportStart+s.delta < 0 {
+			// Adjust normalizedDragOffset - and thus the future s.oldDragPos - so that futile dragging up has to be
+			// countered with dragging down again. Otherwise, dragging up would have no effect, but dragging down would
+			// immediately start scrolling. We want the user to undo their ineffective drag first.
+			normalizedDragOffset -= viewportStart + s.delta
+			// Limit s.delta to the maximum amount scrollable
+			s.delta = -viewportStart
+		} else if viewportEnd+s.delta > 1 {
+			normalizedDragOffset += (1 - viewportEnd) - s.delta
+			s.delta = 1 - viewportEnd
+		}
 		s.oldDragPos = normalizedDragOffset
 	}