Use BBoxLeft, BBoxRight, BBoxTop, BBoxBottom expressions to position your panels.
"On collision" event is a bad choice here anyway, because collision checks are performed every tick and objects can travel big distances in one tick if the speed is high or FPS is low. So, for example if you are dragging a panel and want to stop when it collides with another panel, if you move your mouse too fast, you can easily drag it 100-200px too far.