Preserving minimum sizes with multiple WPF grids and splitters - wpf

Question

I have a problem getting an arrangement of grids, splitters and an expander to work correctly, especially regarding setting min heights. I’ve followed a number of suggestions posted here (especially regarding how to combine a splitter and an expander), and have got the basic architecture working, but I can’t get the final details to work right. I’m sure that I could get something working by trial-and-error, especially if I do some brute force work in the code-behind, but I’m guessing there are some fundamental principles of how WPF resizing works that I’m missing here.
The following xaml is a simplified version of the actual structure, with colors and borders added for clarity. I’ve removed my attempts at setting MinHeights. (There is currently no code-behind.)
<Page
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Width="450"
Height="600">
<Grid Background="Yellow">
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<TextBox x:Name="Tree" Grid.Row="0" Text="Tree" BorderThickness="2" BorderBrush="Red" />
<Expander IsExpanded="True" Grid.Row="1" Header="Expand/Collapse Details" ExpandDirection="Down"
BorderThickness="2" BorderBrush="Black" VerticalAlignment="Bottom">
<Expander.Content>
<Grid x:Name="LowerAreaGrid" Margin="4">
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<GridSplitter HorizontalAlignment="Stretch" Grid.Row="0" Height="8" Background="DarkGray"
ResizeDirection="Rows" ResizeBehavior="CurrentAndNext" />
<Grid x:Name="DetailsPaneAndSubpane" Margin="4" Grid.Row="1">
<Grid.RowDefinitions>
<RowDefinition Height="*" MaxHeight="{Binding ActualHeight, ElementName=DetailsPane}"/>
<RowDefinition Height="Auto" />
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<ScrollViewer ScrollViewer.VerticalScrollBarVisibility="Auto" Grid.Row="0">
<TextBox x:Name="DetailsPane" Text="This is the Details Pane. It is 150 pixels high. If the available space is too small, it will grow a scrollbar to see all the content."
TextWrapping="Wrap" BorderThickness="4" BorderBrush="Green" Height="150"/>
</ScrollViewer>
<GridSplitter Grid.Row="1" HorizontalAlignment="Stretch" Height="8" Background="Brown"
ResizeDirection="Rows" ResizeBehavior="PreviousAndNext" />
<Border BorderThickness="4" Grid.Row="2" BorderBrush="Blue">
<Grid x:Name="SubpaneAndButtons" Margin="4" Grid.Row="1">
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<TextBox Grid.Row="0" Text="This is the Details Subpane. It can grow and shrink."
BorderThickness="2" BorderBrush="Blue"/>
<Grid x:Name="SubpaneButtons" Grid.Row="1" Background="Blue" Margin="4">
<Button Width="200">Do something with subpane</Button>
</Grid>
</Grid>
</Border>
</Grid>
</Grid>
</Expander.Content>
</Expander>
<Grid x:Name="CommandButtons" Grid.Row="2" Background="Pink" Margin="4">
<Button Width="200">Do something with whole window</Button>
</Grid>
</Grid>
</Page>
As you can see, the window has two main areas. There is a tree in the top pane, and when an item in the tree is selected, the bottom area (which is contained in an expander) shows details about the item. There is a splitter that can change the relative sizes of the top and bottom areas. The details area is further divided into two parts: a Details Pane which has a fixed maximum size, and a bottom Details Subpane which can grow and shrink. A second splitter controls the relative sizes of the Details Pane and Details Subpane.
There’s also a button at the bottom of the window that represents a set of buttons that are commands that apply to the whole window. There’s a button at the bottom of the Details Subpane that represents a set of buttons that are commands that apply only to the Details Subpane.
Here’s what doesn’t work:
Problem: the second (brown) splitter can’t be moved up initially. (It’s not expected to be able to move down, since the Details Pane is at its maximum size.) You have to move the gray splitter a little bit, then the brown splitter works fine even if you move the gray splitter back to where it was.
The Tree should have a MinHeight of 75. Problem: putting this in the tree’s RowDefinition causes the rest of the content to slide off the end of the window when you drag the slider up past the 75 pixel mark. (If you don’t have a MinHeight, this sliding occurs when the splitter is dragged up past the top of the window.)
The Details Pane and the Details Subpane should have a MinHeight of 50. Problem: moving either of the splitters down can squish these areas down to nothing.
Neither splitter should affect the “Do something with subpane” button row. It should always be fully visible, unless the expander is collapsed. Problem: again, moving either of the splitters down can squish this area down to nothing.
Here’s what works, and should keep working after all changes are made:
When the “Expand/Collapse Details” expander is collapsed, it collapses everything but the “Do something with whole window” button row. The Tree takes all the remaining space.
The Details Pane is a fixed size (150 pixels). It can’t grow larger than that, and if it is given less space than that, it will grow a scrollbar to allow the entire 150 pixels to be seen still.
What changes need to be made (either in xaml or code-behind) to get this to work?