After changing the style (theme), when you pin a side bar it will disappear

[mladdaga]

After hours of research I found that in LayoutAnchorSideControl, in the LayoutAnchorSideControl_Unloaded method, for some reason, it's removed the collectionChanged handler on the model's children
_model.Children.CollectionChanged -= OnModelChildrenCollectionChanged;
Removing this line from the code everything works fine.
It seems that is the root of a lot of other Issues opened around the load of a different theme.

It's easy to reproduce on the MVVMTestApp

[雲可呀]

Hi mladdaga. Like you said, change the style (theme) and pin a side bar it will disappear.
This is due to the parent's template change, And the child control is disconnected (The Unloaded event handler will be raised).

ApplyNewTemplate (Change the template) -> Unloaded (The event handlers will be removed) -> Loaded (Reconnect, but we don't have the Loaded event handler)

AvalonDock/source/Components/AvalonDock/Controls/LayoutAnchorSideControl.cs

Lines 145 to 149 in 1d4d30d

 private void LayoutAnchorSideControl_Unloaded(object sender, RoutedEventArgs e) 
 { 
 	_model.Children.CollectionChanged -= OnModelChildrenCollectionChanged; 
 	Unloaded -= LayoutAnchorSideControl_Unloaded; 
 } 

[雲可呀]

Fortunately, we can add a Loaded event handler in OnApplyTemplate. So that we can add back the OnModelChildrenCollectionChanged event handler on apply the new template.

// LayoutAnchorSideControl.cs

public override void OnApplyTemplate()
{
	base.OnApplyTemplate();

	Loaded += LayoutAnchorSideControl_Loaded;
}

private void LayoutAnchorSideControl_Loaded(object sender, RoutedEventArgs e)
{
	Loaded -= LayoutAnchorSideControl_Loaded;
	Unloaded += LayoutAnchorSideControl_Unloaded;
	_model.Children.CollectionChanged += OnModelChildrenCollectionChanged;
}

private void LayoutAnchorSideControl_Unloaded(object sender, RoutedEventArgs e)
{
	_model.Children.CollectionChanged -= OnModelChildrenCollectionChanged;
	Unloaded -= LayoutAnchorSideControl_Unloaded;
}

It will work.