How to mimic expanders in Expression Blend

They say imitation is the sincerest form of flattery. I love the UI in
Blend and would love to mimic the way the expand/collapse panels work
(Trigggers, Objects and Timeline, Files, etc). I can't quite figure out how
to get expanders to work this way.

In my attempts, I created a grid, with a GridSplitter and an Expander
control wrapped around a border as
follows (attributes removed for brevity):

<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="50*/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Border>
<Expander Grid.Row="0"/>
</Border>
<Border>
<Expander Grid.Row="1"/>
</Border>
<GridSplitter/>
</Grid>

When the app starts up, everything looks beatiful and works just peachy.
However, as soon as you move the GridSplitter, things start to go haywire.
The border that surrounds the expander no longer expands or collapses when
the expander does. I'm guessing that this is because the the GridSplitter
modifies the ActualHeight of the cells to be actual values instead of keeping
them relative. Is this a correct assumption?

Given this, can someone from MS be kind enough to share the secret to making
this behaviour or does MS consider this some sort of proprietary intellectual
property that must be guarded at all costs?

Thanks for the help.


--
New to EID and loving it