In single-thread view, nesting is at most two

Some time in the last few months, single-thread view stopped nesting deeper than two, for me, whether logged in or not, on all browsers I use, including various Chromes.

Here is an example thread:

https://lists.apache.org/thread.html/04b9a8a156939adae1aa4cf2d4e3abd2137f541cbf92ade6d775dc06@%3Cdev.parquet.apache.org%3E

When viewing this thread in the multi-thread view https://lists.apache.org/list.html?dev@parquet.apache.org, nesting works properly, showing many levels of depth by use of the colored bars on the left-hand side.

Reproduce the reported behavior with the supplied single-thread example, then compare it with the multi-thread view linked in the issue. Trace the frontend path responsible for rendering nested replies and identify why single-thread view stops at depth two. Done means deeper nesting is shown consistently, while the multi-thread view remains correct.

Written by the indexing model from the issue text.