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LayoutGroup constraint issues
There is an issue with your code in the LayoutPanel:
public class LayoutPanel : LayoutPositionableGroup<ILayoutPanelElement>, ILayoutPanelElement, ILayoutOrientableGroup
|=> public abstract class LayoutPositionableGroup<T> : LayoutGroup<T>, ILayoutPositionableElement, ILayoutPositionableElementWithActualSize where T : class, ILayoutElement
|=> public abstract class LayoutGroup<T> : LayoutGroupBase, ILayoutContainer, ILayoutGroup, IXmlSerializable where T : class, ILayoutElement
Now I don't know why you added the requirement for "where T : class" because I never see it being instantiated. I am even confused how this can compile. Anyway, at runtime you get this error:
GenericArguments, 'Xceed.Wpf.AvalonDock.Layout.ILayoutPanelElement', on 'Xceed.Wpf.AvalonDock.Layout.LayoutGroup`1[T]' violates the constraint of type 'T'."
I think it is best to remove the where T : class (but keep the where T : ILayoutElement) because I think it is not required.