Root index.html deleted unpredictably

The static index.html and index.html.gz are deleted unpredictably. This happens when the Cache rebuild option is checked and it appears to be triggered by posting to a php-file in the root directory.

There are several mentions in the support forums.

Start by tracing the Cache rebuild option and the handling of root index.html and index.html.gz, then reproduce the deletion while posting to a PHP file in the root directory. Use the linked support reports as reproduction context; done means rebuilding the cache and posting to that PHP file no longer unpredictably deletes either static index file.

Written by the indexing model from the issue text.