Currently the past of message is traced with message only. It causes a side effect of a 'history merge' in following snippet:

case msg =>
  actor1 ! traced_message
  actor2 ! traced_message

Briefly, when I move toward future I send message to actor1 only (I trace this time line only). But then when I try to go back into the past I see that in my time line both sendings took place.

It seems that the sender and triggering message should be included into message flow history. It won't cover the following case:

case msg =>
  actor1 ! traced_message
  ...
  actor1 ! traced_message

we won't be able from where actually the message has been sent but for message flow it does not matter.

Remark:
test following snippet:

case msg1 =>
  actor1 ! traced_message
case msg2 =>
  actor1 ! traced_message

As a result these two time lines should be separate.