do not drag out of UI bar

bitplane-thingie.p8

@@ -35,6 +35,11 @@ function _update60()
   return
  end
 
+ -- ignore drag off ui
+ if was_click and segment == nil then
+  return
+ end
+
  -- mouse down
  local mouse_x = stat(32)
  local mouse_y = stat(33)