We don't want to make breaking visual changes to our existing controls as this will affect many of our clients. Furthermore the highlight effect of TreeViewItem-s i.e. the MouseOver visual state was introduced in order to be consistent with the native WPF TreeView control and provide better user experience.
Unfortunately there is no other way to remove the MouseOver visual element from the control's default template but to modify it manually as my colleague Tina explained. Now it is easier than ever to do this -- just Extract the control template via VisualStudio Designer or Expression Blend and do the needed change. We have help article that explains how to extract and edit our controls Control Templates here
Let me know if there is anything else I can assist you with.
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