src/vs/workbench/contrib/files/browser/views/openEditorsView.ts/OpenEditorsView/getIndex
class OpenEditorsView:	private getIndex(group: IEditorGroup, editor: IEditorInput | undefined | null): number {
		if (!editor) {
			return this.elements.findIndex(e => !(e instanceof OpenEditor) && e.id === group.id);
		}

		return this.elements.findIndex(e => e instanceof OpenEditor && e.editor === editor && e.group.id === group.id);
	}