Bug: GUI squashes deeply nested conversations

Something is wrong with the layout of threaded mode.

If a conversation is deeply nested (e.g. [1]), the RHS margin of the message text is gradually moved leftwards, reducing the usable space.

It looks like additional padding is being added to the RHS every time another level is added.

[1] https://lists.apache.org/list.html?users@httpd.apache.org:lte=24M:mod_lua

Start by reproducing the deeply nested conversation described in the linked Apache mailing-list example and inspect the GUI's threaded-mode layout. Trace how each nesting level affects the message text's right-hand margin; done means deeply nested conversations retain usable text width without accumulating right-side padding.

Written by the indexing model from the issue text.