>  Wouldn't it be worth to clean all weakmaps to avoid headaches for someone introducing such new cycles?

I'm afraid it's not that easy, as system event notifications are not GC-ed immediately also if user holds refs to panel then it still needs to ask questions about it's width / height, etc... Maybe users should not be able to do so, but we have tests assuming so and I don't wanna make too many changes.

Also one of the goals is to reduce cleanup work to bare minimum to speedup unloads. Once @zero will land WeakSet's we won't even have to do this, instead we'll let GC do the work.