strange behavior when editing sequence diagrams (#433)

When I work on a sequence diagram, I start getting "strange" effects after a while, which make it difficult to work on the diagram.

Example:

- I cannot make some execution occurence blocks smaller than a certain height and I don't know why. Sometimes they are quite long / large (about 300px), and I cannot make them smaller.

- I therefore have to delete and recreate the execution occurence block. Now all of the incoming / outgoing messages of that block are "floating around" without being connected to the new block. I cannot find a way to reconnect them to the block. Often I end up with deleting and recreating them.