src/vs/editor/browser/editorDom.ts/DynamicCssRules/getOrCreateRule
class DynamicCssRules:	private getOrCreateRule(properties: CssProperties): RefCountedCssRule {
		const key = this.computeUniqueKey(properties);
		let existingRule = this._rules.get(key);
		if (!existingRule) {
			const counter = this._counter++;
			existingRule = new RefCountedCssRule(key, `dyn-rule-${this._instanceId}-${counter}`,
				dom.isInShadowDOM(this._editor.getContainerDomNode())
					? this._editor.getContainerDomNode()
					: undefined,
				properties
			);
			this._rules.set(key, existingRule);
		}
		return existingRule;
	}