All the documentation seems to suggest that setting the RenderTransform isn't supposed to impact arrange. But that's not exactly the behavior I'm seeing.
I have an odd requirement for a control layout. I wanted to leave as much of wpf's layout system as I could, so I created an element that inherits Decorator.I have an attached property that marks the elements I'm interested in. During arrange, I call the base ArrangeOverride method, which layouts out the elements using the normal process. Next, I crawl through the controls with the attached attribute and, using the render transform, I re-position the controls where I want them.
All of this works fine, until I use an items control. Assigning the RenderTransform is causing the arrange to be invalidated and my arrange method is being called repeatedly. The documentation seems to suggest setting the RenderTransform has no impact on Arrange.