How to wait for the WPF render thread to "catch up"?

In WPF, there is both your UI thread and the "hidden" render thread. As you make changes to your controls in the UI thread, those are not reflected immediately. The render thread has to render those changes to the display.

How does one block the UI thread until the render thread completes all of its pending tasks? I would like the render-thread equivalent to GC.WaitForPendingFinalizers(). Where is the call WPF.WaitForPendingRenders()?

Wally