Hi Jerry thanks for your post! Have you tried switching off line connections in your diagram? This allows you to create lines that move independently without connecting to other lines and shapes so it should give you more control over the path and placement of your lines.

To create a line that moves independently from a shape toggle off "line connections" in your Page Settings (File > Page Settings) on the right of your canvas. However one thing to note is that lines that already have connections will need to be disconnected for this setting to apply. 

For more information about modifying the path of your lines I recommend checking out this section of our Work with Lines article. Hope this helps! Please let me know if you have any questions.

I have the exact same problem. A "loopback" line is extremely common in UML sequence diagrams and it seems that the only way to add one is to disable line connections for the entire diagram? Is that really true?

Hi Keith thanks for continuing this thread. If you are using markup to build your sequence diagram you can create a loopback line using the markup itself using "participant -> participant" as shown in this example below:

However if you are manually building your sequence diagram and wish to customize your loopback lines (like what Jerry describes above) it is necessary to disable line connections as this is the functionality that causes lines to "reach for" and "stick to" adjacent shapes and lines. Apologies for any inconvenience this causes! Please let me know if you have additional questions.