src/vs/workbench/browser/parts/editor/sideBySideEditor.ts/isSideBySideEditorViewState
function isSideBySideEditorViewState(thing: unknown): thing is ISideBySideEditorViewState {
	const candidate = thing as ISideBySideEditorViewState | undefined;

	return typeof candidate?.primary === 'object' && typeof candidate.secondary === 'object';
}