Sequence Diagram : Activations via messages

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Hi

I'm creating a sequence diagram and use messages to start and stop activations of the involved Lifeline objects.

I'm using the option "Activations" in the context-menue of Message-objects, but here I'm missing the option "Extend target activation down" which would help in some problems.

Generally I'm having several problems to get the activation rectangles to look exactly as I want to.

Is there a direct way to control the activation rectangles.

Thanks
Frank

P.S.:
I have created the diagram out of scratch, i.e. no source code or anything else is connected.