message: Fix the color picker popup position (win)
diff: <Popup Name="popup" x:FieldModifier="private"
  MaxHeight="300" StaysOpen="True" AllowsTransparency="True"
  PlacementTarget="{Binding ElementName=button}"
  - Placement="Left"
  - HorizontalOffset="{Binding ActualWidth, ElementName=button}"
  - VerticalOffset="{Binding ActualHeight, ElementName=button}"
  + Placement="Bottom"
  PreviewKeyDown="onPopupPreviewKeyDown"
  IsKeyboardFocusWithinChanged="onPopupKeyboardFocusWithinChanged">
  