Effects are greyed out in the effects panel unless an effect in the panel is already pre-selected

“If a layer is not highlighted, there is no target for After Effects to apply effects to.”

Sure there is—the target is the un-highlighted layer whose effects are still listed in the effects panel.

The design language implies I can still interact (e.g., add another effect) with the effects panel in its current state. Otherwise why are effects still listed when no layers are highlighted? If clicking the name of any effect applied to an un-highlighted layer SELECTS the layer for me in the timeline (at which point I can add an effect), then so should right clicking in the panel - it’s not ambiguous what layer a newly-applied effect would be attached to. It’s being applied to the un-highlighted layer whose effects are still listed and I’m right-clicking next to.