It is my understanding that the OnMouseDown function is called for whatever collider or GUI element is under the mouse when the button is clicked. However, it appears to be the case that only the first element hit has its function called, and none of the subsequent elements that may be under the mouse. Is it possible to cause other elements to also have this function called for them, in a sense changing the logic from a Raycast type test to more of a RaycastAll type of test?
You have to write your own code using RaycastAll; there isn’t a way to change built-in behaviors.
You could try using “HitTest” instead…
http://unity3d.com/support/documentation/ScriptReference/GUILayer.HitTest.html?from=GUIElement