I have a simple simulation where agents have a field that is set initially at a 
random number and then increases by one with every step. The portrayal showing 
the value of this field shows the values growing in colour intensity until the value 
crosses the maximum value set in the SimpleColorMap class constructor. 

If the simulation is run, stopped and run again, everything is fine. But if at any 
point the "Show Layers" button is pressed, regardless of whether or not any of 
the checkboxes indicating the layers are checked or unchecked, the simulation 
slows down until the agent field values cross the maximum value. As each agent 
reaches this maximum the simulation starts to speed up again, and once all 
agents have crossed it the simulation runs as normal. It will still do this even 
when it is stopped and restarted again. Simply clicking on that button once will 
have this effect until the whole application is closed down.

Why does clicking on this button have this effect?