One of my users noticed that while "INBOX" appears in the list of mailboxes listed under:

Settings => Preferences => Special Folders => Sent

it is not possible to select that folder for sent messages. Any attempt to do do ends up with the empty value "---".

A bit of digging uncovered program/steps/settings/func.inc:

$select = rcmail_mailbox_select(array( 'noselection' => '---', 'realnames' => true, 'maxlength' => 30, 'folder_filter' => 'mail', 'folder_rights' => 'w', // #1486114, #1488279 'onchange' => "if ($(this).val() == 'INBOX') $(this).val('')", # ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ));

http://trac.roundcube.net/ticket/1486114 explains the problem with Drafts, which make sense. I have pointed the user in question at

Settings => Composing Messages => Place replies in the folder of the message being replied to

but that isn't quite what they are looking for: they want incoming and outgoing email in a single mailbox (this is a shared account).

This is with Roundcube 0.9.2, although I don't think that is particularly significant. The ticket that I refer to above is a few years old now.