src/vs/base/common/observableImpl/derived.ts/Derived/handleChange
class Derived:	public handleChange<T, TChange>(
		_observable: IObservable<T, TChange>,
		_change: TChange
	): void {
		if (this.hasValue) {
			this.hadValue = true;
			this.hasValue = false;
		}

		// Not in transaction: Recompute & inform observers immediately
		if (this.updateCount === 0 && this.observers.size > 0) {
			this.get();
		}

		// Otherwise, recompute in `endUpdate` or on demand.
	}