src/vs/platform/layout/browser/zIndexRegistry.ts/ZIndexRegistry/registerZIndex
class ZIndexRegistry:	registerZIndex(relativeLayer: ZIndex, z: number, name: string): string {
		if (this.zIndexMap.get(name)) {
			throw new Error(`z-index with name ${name} has already been registered.`);
		}

		const proposedZValue = relativeLayer + z;
		if (findBase(proposedZValue) !== relativeLayer) {
			throw new Error(`Relative layer: ${relativeLayer} + z-index: ${z} exceeds next layer ${proposedZValue}.`);
		}

		this.zIndexMap.set(name, proposedZValue);
		this.scheduler.schedule();
		return this.getVarName(name);
	}