on openStack send "rollCredits" to field "Credits" end openStack
The OpenStack message is sent to the current card, but if it isn't the card, it's sent to the stack. So if you put the Openstack both in stack and in card, only the one in the card will be executed.
The openStack message is sent after the stack is visible.
To make changes that take effect before the stack becomes visible on the screen, use a preOpenStack handler instead of openStack.
The actual opening is not triggered by the openStack message, so trapping the message and not allowing it to pass does not prevent the stack from opening.