This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
renderLanes,
        );
        workInProgress.memoizedState = SUSPENDED_MARKER;
        return fallbackChildFragment;
      } else {
        const nextPrimaryChildren = nextProps.children;
        const primaryChildFragment = updateSuspensePrimaryChildren(
          current,
          workInProgress,
          nextPrimaryChildren,
          renderLanes,
        );
        workInProgress.memoizedState = null;
        return primaryChildFragment;
      }
    } else {
      
      if (showFallback) {
        
        const nextFallbackChildren = nextProps.fallback;
        const nextPrimaryChildren = nextProps.children;
        const fallbackChildFragment = updateSuspenseFallbackChildren(
          current,
          workInProgress,
          nextPrimaryChildren,
          nextFallbackChildren,
          renderLanes,
        );
        const primaryChildFragment: Fiber = (workInProgress.child: any);
        const prevOffscreenState: OffscreenState | null = (current.child: any)