WPF UserControl flickering when moving mouse outside during command processing

I have written a WPF UserControl (let's called it ContainerUserControl), inside it it uses other UserControl (let's call it OstUserControl) which I build from another project. Within the ContainerUserControl, I have a RibbonGroup hosting bunch of RibbonButtons. The OtsUserControl can respond to user mouse clicks on a toggle button inside and runs the same code if user clicks a corresponding RibbonButton.

However the odd thing is: If I click the RibbonButton (which is outside of the border of the OtsUserControl), there is flickering around the boundary of OtsUserControl; If I click a toggle button inside OtsUserControl and don't move the mouse outside the OtsUserControl, there is NO flickering; If I click a toggle button inside OtsUserControl and move the mouse outside the OtsUserControl, there is flickering.

How can I fix this flickering problem?

BTW, the code to invoke it via RibbonButton goes through MVVM Light's RelayCommand mechanism.

Jun HY