I am adding comments from outside the sequence diagram onto 2 frames and the arrow doesn't go past the frame boundary. Arrow from decorations palette needs to span across frames. Thanks.
You can do it by floating the decoration using the same trick one uses to float a control on top of a Tab Control.
1. Place the arrow on the Block Diagram (not in the Sequence Structure)
2. Select it and move it over the SS using the arrow keys (shift helps move it faster).
3. Now the decoration is floating and not owned by the SS, you can resize and reposition.
Of course one element missing from the Tab control trick is grouping which for some strange reason is not allowed on the block diagram. That just means the two items (SS + arrow) move independently so you may have to do some shifting later on.
