Why are some headers not included by Engine.h?

Some header files seem to be missing from Engine.h, such as UDecalComponent. Is there a reason for that?

Engine.h includes EngineComponentClasses.h, which in turn includes most of the classes in
Source/Runtime/Engine/Classes/Components, except a few, such as UDecalComponent.

If this is by design, what is the recommended way to include these classes? I’ve seen an other thread regarding
ParticleSystemComponent, and the solution was to include ParticleDefinitions.h. If there are other similar cases, a hint in the API docs would be a good idea, I think.

As explained here, this is by design.

I found that EngineDecalClasses.h includes DecalActor.h and DecalComponent.h. Including that fixed my problem.