This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
renderLanes,
            );
            const primaryChildFragment: Fiber = (workInProgress.child: any);
            primaryChildFragment.memoizedState = mountSuspenseOffscreenState(
              renderLanes,
            );
            workInProgress.memoizedState = SUSPENDED_MARKER;
            return fallbackChildFragment;
          }
        }
      }
      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)
          .memoizedState;
        primaryChildFragment.memoizedState =
          prevOffscreenState === null
            ? mountSuspenseOffscreenState(renderLanes)
            : updateSuspenseOffscreenState(prevOffscreenState, renderLanes);
        primaryChildFragment.childLanes = getRemainingWorkInPrimaryTree(
          current,