The DisplayListBuilder used to only provide a symmetric interface to the DlOpReceiver methods, thus Receiver methods in, Receiver methods out.

At one point the DlCanvas API was created that is much more developer friendly and the Builder was modified to take those methods in addition to the Receiver methods. Later, the Builder's Receiver variant methods were hidden so that public uses of it would all go through its more friendly DlCanvas API and the old Receiver methods continued on only for use in many unit tests that exercise them for internal purposes.

But, many DisplayList unit tests aren't testing the internal state of the Builder and so they should be modified to only use the public DlCanvas methods on the Builder to test their operations. We may not reach 100% conversion to testing via the public methods, but that should be the goal to reach for.