Commented Unassigned: AvalonDock: Left/right docking is broken when LayoutAnchorable is docked to LayoutDocument [22492]

Simple repro XAML:

```
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApplication1"
xmlns:xcad="http://schemas.xceed.com/wpf/xaml/avalondock"
mc:Ignorable="d"
Title="MainWindow"
Height="350"
Width="525">
<Grid>
<xcad:DockingManager>
<xcad:LayoutRoot>
<xcad:LayoutRoot.RootPanel>
<xcad:LayoutPanel Orientation="Vertical">
<xcad:LayoutDocumentPaneGroup>
<xcad:LayoutDocumentPane>
<xcad:LayoutDocument Title="Document1">
<TextBlock Text="Document1" />
</xcad:LayoutDocument>
</xcad:LayoutDocumentPane>
</xcad:LayoutDocumentPaneGroup>
<xcad:LayoutAnchorablePaneGroup>
<xcad:LayoutAnchorablePane>
<xcad:LayoutAnchorable Title="Anchorable1">
<TextBlock Text="Anchorable1" />
</xcad:LayoutAnchorable>
</xcad:LayoutAnchorablePane>
<xcad:LayoutAnchorablePane>
<xcad:LayoutAnchorable Title="Anchorable2">
<TextBlock Text="Anchorable2" />
</xcad:LayoutAnchorable>
</xcad:LayoutAnchorablePane>
</xcad:LayoutAnchorablePaneGroup>
</xcad:LayoutPanel>
</xcad:LayoutRoot.RootPanel>
</xcad:LayoutRoot>
</xcad:DockingManager>
</Grid>
</Window>
```

Repro steps:
* Move "Anchorable1" pane up so that it's docked with "Document1" (and internally it appears to convert from an Anchorable to Document
* Try to dock "Anchorable2" pane to "Anchorable1" pane on the left or right side and note that the left/right arrows are missing.

This can be worked around but it's rather clunky:

* Make "Anchorable1" pane floating
* Dock "Anchorable2 to Anchorable1's left or right side
* Now dock the combined Anchorable1/Anchorable2 pane to "Document1"
Comments: ** Comment from web user: theficus **

AllowMixedOrientation seems to do what I want. Thanks!