XamlParseException using Extended.Wpf.Toolkit in VS extension project

I have an existing WPF application from which I'd like to make a VisualStudio extension.

Basically, I have a xaml window in which I use the library Extended.Wpf.Toolkit for AvalonDock. This application works perfectly without any issue.

I tried to re-use the same window in my extension project and I got XamlParseException on load

Here is the sample code which fails :

<Window x:Class="Company.VisualStudioExtension.Window2"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:xcad="http://schemas.xceed.com/wpf/xaml/avalondock"
    Title="Window2" Height="300" Width="300">
<Grid>
    <xcad:DockingManager AllowMixedOrientation="True" BorderThickness="1">
        <xcad:DockingManager.DocumentHeaderTemplate>
            <DataTemplate>
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="Test" />
                </StackPanel>
            </DataTemplate>
        </xcad:DockingManager.DocumentHeaderTemplate>
        <xcad:LayoutRoot x:Name="_layoutRoot">
            <xcad:LayoutPanel Orientation="Horizontal">
                <xcad:LayoutAnchorablePane DockWidth="300">
                </xcad:LayoutAnchorablePane>
            </xcad:LayoutPanel>
        </xcad:LayoutRoot>
    </xcad:DockingManager>
</Grid>

The exception is highlighted on the following line :

            <xcad:LayoutPanel Orientation="Horizontal">

"The method or operation is not implemented."

EDIT

It seems registering all AvalonDock dll in GAC fixes the issue but obviously, it's not an acceptable solution...

I guess these dll are not properly registered while running as an Extension, I propably need to reference them in a specific way... for now, they are referenced in Extension's csproj + CopyLocal=True

EDIT2

Added source code to reproduce the issue https://github.com/MrLuje/VSPackage_NotWorking

There are 2 projects :

a WPF app with a working code in MainWindow
a VS Extension with non-working code in Window1 (in debug, you need to click in Tools > "My Command name")

Solution

I found an easier solution, 'ProvideBindingPath' this will add your current extension folder to the resolution folders of dlls.

 [ProvideBindingPath]
 public sealed class MainWindowPackage : Package
 {
 }

based on solution purposed in: VSIX - Cannot load file or assembly of a referenced dll

The solution of adding to the gac wont work 'as is'...VSIX extensions dont allow register on the gac, maybe a workaround will be run gacutil.