src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/setIndex
class EditorGroupModel:	setIndex(index: number) {
		// We do not really keep the `index` in our model because
		// it has no special meaning to us here. But for consistency
		// we emit a `onDidModelChange` event so that components can
		// react.
		this._onDidModelChange.fire({ kind: GroupModelChangeKind.GROUP_INDEX });
	}