Corrupt output with nested spans

I've noticed that dindent was modifying text in my string. After some testing I've found out that the simplest input on which it fails is:
``
It produces:
``

Reproduce the corruption with the nested-span input from the issue and compare the output with the original text. Trace the dindent processing entry point for this HTML, then add a regression test showing that the input is preserved rather than duplicated. Done means the failing example produces unchanged text.

Written by the indexing model from the issue text.