I have seen this several times when using a Container Tool on the Canvas. If the title bar of the container tool goes outside the Canvas workspace, such as if it expanded when tools were placed inside or was moved inadvertantly, it can be hard or near impossible to select it to bring it back down, configure it, etc. Check out the screenshot and you'll see half of an empty tool container in the Canvas.
Zooming out does not make a difference and utilizing the Overview window doesn't help either.
I can duplicate this issue rather easily and haven't found an easy way to prevent this issue from happening. It seems as if the Canvas boundaries should prevent a tool container from going outside the top most border. Anyone else run into this?