src/vs/workbench/common/editor.ts/isResourceSideBySideEditorInput
export function isResourceSideBySideEditorInput(editor: unknown): editor is IResourceSideBySideEditorInput {
	if (isEditorInput(editor)) {
		return false; // make sure to not accidentally match on typed editor inputs
	}

	if (isResourceDiffEditorInput(editor)) {
		return false; // make sure to not accidentally match on diff editors
	}

	const candidate = editor as IResourceSideBySideEditorInput | undefined;

	return candidate?.primary !== undefined && candidate.secondary !== undefined;
}