If i have a Long running Process inside a UI (I know it should not run long).
Should the Heartbeat come through or not?

It seams it does not?
this means that the heartbeat did not differe between a running UI and a missing UI.

Also there is no more then One Heartbeat if i for example stop the current Call inside a debug breakpoint.
I see only one Heartbeat call to the server and no more. there should at least be 3 (config) and then a break.

Vaadin 7.4.0

Yes, heartbeat can’t get through because only one request per session can be active at a time (the session is locked to prevent concurrency issues). This is one of the reasons one should not have long-running requests as you mentioned.

A new heartbeat is scheduled on the clientside only after another is finished, so if one stalls indefinitely, others won’t get sent.