Playing around with the activity diagrams and swimlanes, I found a bug with how fork and swimlanes are shown. It seems the bar for a fork won't cross the swimlane. Here's a simple example
@startuml
|Swimlane1|
start
:foo1;
fork
|#AntiqueWhite|Swimlane2|
:foo2;
|Swimlane1|
forkagain
:foo2.1;
end fork
|Swimlane2|
:foo5;
stop
@enduml