src/vs/workbench/browser/parts/panel/panelPart.ts/BasePanelPart/removeNotExistingComposites
class BasePanelPart:	private removeNotExistingComposites(): void {
		const panels = this.getPaneComposites();
		for (const { id } of this.getCachedPanels()) { // should this value match viewlet (load on ctor)
			if (panels.every(panel => panel.id !== id)) {
				this.hideComposite(id);
			}
		}
	}