I would like to have the Expander from question below (see the accepted answer) which is a perfect solution for me. I just want to have it on the right side.
Combine expander and grid (resizable expander)
To have it expanded from right to left I changed the settings for an analoguous behavior. (I emphasized elements to see better what's going on):
<Expander Grid.Column="1" Header="Test" ExpandDirection="Left"
BorderThickness="10" BorderBrush="Black" HorizontalAlignment="Right" >
<Expander.Content>
<Grid >
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<GridSplitter Grid.Column="0" Width="10" Background="Green"
ResizeDirection="Columns" ResizeBehavior="CurrentAndNext" />
<TextBox Grid.Column="1" Text="Lorem ipsum dolor sit"
BorderThickness="10" BorderBrush="Red"/>
</Grid>
</Expander.Content>
</Expander>
It resizes the area left from the splitter when I move the splitter to the right. I tried many other combinations but it almost resulted in the same unwanted behavior. The area left of the splitter is sometimes kind of weirdly exploding but TextBox remains unchanged.
PreviousAndCurrent
asResizeBehavior
? – Daniel Hilgarth