The Canvas rendering code has to special case several rendering attributes to combine results from intermediate buffers. These attributes include BlendMode, clipping, and effects. Those combined operations are further complicated by the GlobalAlpha and the transform.

Combining one or more of those operations can lead to wrong results or exceptions in the worst cases. The two attached test cases can be used to explore the combinations that fail. (The Combined test case requires an HD screen to see the entire window, but is faster to cycle through all combinations of the operations and all shape types by clicking on the window until it cycles through after a dozen clicks or so.)