If a selected pool comes to the foreground thereby hiding other diagram elements, the nodes that become hidden have not been properly assigned to the pool. Try selecting the pool's nodes (but not the pool itself), then press and hold the SHIFT key while moving the nodes over the pool. (If the nodes already lie over/inside the pool, just move them around a bit with SHIFT down, but make sure they remain inside.) The next time you accidentally select the pool, its nodes will remain visible.
You can easily check whether or not a diagram node is assigned to a pool by selecting it and trying to move it outside the pool. If the node is assigned to the pool, the pool will automatically grow to encompass the node thereby making it impossible to move the node out of the pool.
A node can be detached from its pool by holding down the SHIFT key while moving the node out of the pool.
