Whether the open design currently builds into a simulation, which is all the Simulation tab needs in order to decide whether its button reads "Start Simulation" or "See Problems", and whether to offer the waveform simulator. It is here, in the model, because answering it means flattening the whole hierarchy: the tab used to ask on every render, so a large design was flattened again for every frame it was visible. Now it is answered once per edit, on a delay, and the last answer stands while a new one is worked out - a button that is briefly a moment out of date, in exchange for an editor that does not stop. The verdict is reached by validateCircuitSimulation, which builds the graph and no FastSimulation, so answering it allocates no step arrays.