Currently RenderMarquee is not a RenderObject and hangs off RenderLayer. It should be the other way around ie it's a RenderObject with a RenderLayer (or maybe even without one).
The most delicate part of this change is that we have very little coverage for RenderMarquee and the existing testing is very timing dependant (same issue as animation). We would need to increase the coverage before landing such a refactoring.
Created attachment 160025 [details]
Prototype patch: not for review as it doesn't have proper test coverage
Created attachment 184850 [details]
Updated concept patch: manually tested to work