GUI Event When Overlapping
 

Any GUI container that displays above another and hides itself on the onMouseDown() event will trigger an onMouseUp() event on the container under it, which is expected. However, base GUI controls behave this way and there's really no way for the scripter to fix it.

For example, if you have a GuiPopUpMenuCtrl which when expanded displays above anything waiting for an onMouseUp() event, the onMouseUp() event will be called when selecting a row from the menu.

This should be very simple to fix -- instead of GUI containers such as the GuiPopUpMenuCtrl waiting for an onMouseDown() event, have them wait for the onMouseUp() event so they catch the event instead of anything under them. Besides, this is how drop down boxes work in other applications.

It's not that simple though, a popup control can be opened in different ways, not necessary by a mouse down event. Also the control is itself passing the event when being closed, which is mainly for making it possible to click on other buttons or controls even if the menu is still open. I can check the mouseup event though, to eventually not pass that.