Hello Ogi Ivanov,
1. Currently there is no way to stop the selection from the ViewModel. If you want to use databinding you may create a property in your view that will be databound to a property in your ViewModel that will specify whether the PreviewSelectionChanged event should be handled (this will stop the selection).
The above is also true if you make this into an attached property or behavior and attach it where you want the selection to be handled.
2. The item will be grayed-out if it is selected but the TreeView has no focus. Calling Focus() on the TreeView should keep it unchanged.
Alternatively if you want to have the same visual look but keep the focus away from the TreeView you will need to edit the TreeViewItem's template.
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