'p5 WEB' sometimes does not show

‘p5 WEB’ sometimes does not show.

//-> setup
var cbox;
cbox = select('#map_div'); cbox.position(10,160); cbox.hide();
//-> button event

When the ‘cbox.show()’ operation is performed, the map should be visible, but sometimes it is not visible. Why?
It is invisible, but can be seen by moving the ‘mouse-wheel’ over the area.

  1. I used ‘show’, but the map is not visible.
  2. If you use the ‘mouse-wheel’ on that area, the map will be visible.
    Why? What’s the matter?