title: WPF ComboBox popup placement : Bottom and aligned to the right edge
body: <p>I'm trying to create a <code>ComboBox</code> with a non-standard dropdown alignment. Basically, I want the dropdown to be below the <code>ComboBox</code>, but aligned with the right edge of the <code>ComboBox</code> instead of the left edge.</p>
  
  <p>What a normal <code>ComboBox</code> looks like, using <code>PlacementMode="Bottom"</code>:</p>
  
  <p><a href="https://i.stack.imgur.com/aGmbC.png" rel="noreferrer"><img src="https://i.stack.imgur.com/aGmbC.png" alt="combo box aligned to the left"></a></p>
  
  <p>What I want:</p>
  
  <p><a href="https://i.stack.imgur.com/TnzxI.png" rel="noreferrer"><img src="https://i.stack.imgur.com/TnzxI.png" alt="combobox aligned to the right"></a></p>
  
  <p>I tried to play with the <code>Popup.PlacementMode</code> property in the template of my <code>ComboBox</code>, but none of the possible values seem to do what I want. Is there a simple way to do it, preferably in pure XAML?</p>