WPF pulls the scroll bar throw exception back and forth

The controls, which quickly move the scroll bar back and forth, throw the following exception, but the full code is not captured.

This tree uses the virtualizingstackpanel.isvirtualizing= "True". It's normal to get rid of the sentence. When the data volume of the node is large, the display is very lag, so the attribute must be guaranteed.

Find the problem online, there is no similar answer. So the gradual investigation. The discovery is <separator this separator line produces the anomaly, replaces the border to be possible.

Strangeness, Strange!!!

WPF pulls the scroll bar throw exception back and forth