A bug was logged for this issue about a month ago and a fix will be included in an upcoming patch release -- not sure when that will be, unfortunately. Reviewing the bug, it appears there is some locking issue between line gutter navigation and calculating breadcrumbs. I have not tried any workarounds myself, but the suggestions are either to disable line gutter preference for the code editor, or to avoid moving the (mouse) cursor while breadcrumbs are calculated.
Hope this helps,
SQL Developer Team
Thanks for the update Gary.
A bit more info for you: It also seems to happen with "Comparing PACKAGENAME Body subprograms" (it's happening to me right now)
So the problem may be wider than just the breadcrumbs.
I will try disabling the line gutter preference and see if that helps.
All the best.