Why is OverrideMetadata not responding? - wpf

Question

Simple question. I've subclassed StackPanel as you can see below. I'm trying to change it's default for Grid.IsSharedSizeScopeProperty to true since the default behavior should have that set.
As you can see, I tried overriding the property in the static initializer, but it's not reflecting in the UI. However, if I instead explicitly set it in the instance constructor, it works!
My question is why is the default being overridden not affecting the layout but the explicit setting of it does?
Here's my XAML...
<c:LabeledStackPanel>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" SharedSizeGroup="Label" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<TextBlock Text="Short: " TextAlignment="Right" />
<TextBox Grid.Column="1" />
</Grid>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" SharedSizeGroup="Label" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<TextBlock Text="Medium-size: " TextAlignment="Right" />
<TextBox Grid.Column="1" />
</Grid>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" SharedSizeGroup="Label" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<TextBlock Text="Really long size: " TextAlignment="Right" />
<TextBox Grid.Column="1" />
</Grid>
</c:LabeledStackPanel>
Trying to change the default for the property. This does not work...
public class LabeledStackPanel : StackPanel {
static LabeledStackPanel(){
Grid.IsSharedSizeScopeProperty.OverrideMetadata(
typeof(LabeledStackPanel),
new FrameworkPropertyMetadata(true));
}
}
But this does (so I know the property itself properly affects the layout):
public class LabeledStackPanel : StackPanel {
public LabeledStackPanel(){
Grid.SetIsSharedSizeScope(this, true);
}
}
Ironically, doing both doesn't work either! This one really has me stumped! It's as if trying to set the default somehow breaks the behavior entirely.
public class LabeledStackPanel : StackPanel {
static LabeledStackPanel(){
Grid.IsSharedSizeScopeProperty.OverrideMetadata(
typeof(LabeledStackPanel),
new FrameworkPropertyMetadata(true));
}
public LabeledStackPanel(){
Grid.SetIsSharedSizeScope(this, true);
}
}
So what am I missing here?
My guess is it may have something to do with the following comment from here (although this isn't a template):
Grid size sharing does not work if IsSharedSizeScope is set to true within a resource template, and a SharedSizeGroup is defined outside of that template.
For the record, I have also tried using AddOwner to redefine the property that way, but same thing. Doesn't work.