BUG: frontpage cache is often lost

I've had this bug on different sites for a long time. Now my situation was dire enough (~30 sec per page render) to force me debug it.

Bug: frontpage cache is lost, forcing re-render often.

How to reproduce: disable cache timeout (doesn't matter, just for convenience), enable preload mode with no cache expiration. Enable cache rebuild (important).

Make some or all pages... Mobile github lost my report text. In short words, cache it, invoke cron, frontpage cache lost.

Wrong logic in marking 'to remove dir'.

Please fix.

Start by reproducing the described preload, no-expiration, cache-rebuild configuration, then trace the cron-triggered cache rebuild and the logic that marks directories for removal. Done means the cached front page remains available after cron runs and is not unnecessarily re-rendered.

Written by the indexing model from the issue text.