Thanks Boyko for the response.
Unfortunately, the solution you've suggested to me seems to be a little buggy.
The first time the method is called (I'm expanding the Element in my Form_Load()), the GetCaptionElement().Bounds.Height is returning 16. The caption is actually 32 pixels high - the bottom half of the caption is not visible when the PanelBar is shortened to 16 pixels.
The second time the method is called (I'm manually collapsing the Element), the GetCaptionElement().Bounds.Height is returning 31. This is better, but still off.
The third time the method is called (manually expanding the Element), the GetCaptionElement().Bounds.Height is returning 32. It continues to return 32 on each subsequent activation of the handler.
This isn't particularly urgent. I've actually hard-coded the values I need for the size of the form component into the handler. On collapse, I'm setting it to 32 myself, and on expansion I'm setting it to 183 (32 caption + 150 content + 1 for panelBar border). This is working for me, so I have my solution.
That said, it would have been nice to find a method of doing this that would consistently worked even if the size of that content panel was dynamically changing.
Feel free to let this thread languish for a while - like I said, I've got my form working the way I want. This is now more of a general curiosity thing than anything else.