When using skinparam defaultTextAlignment center in an activity diagram (beta), it correctly centers text on if and else boxes, but not elseif boxes:
@startuml
skinparam defaultTextAlignment center
start
if (condition A \n or \n condition E) then (true)
:do something;
elseif (condition B)
:do something else;
endif
if (condition C \n or \n condition F) then (true)
:do something again;
else (false)
:do another thing;
endif
end
@enduml