MailMate has an unsupported feature for creating custom layouts. I quickly created a combination of the two best (IMO) layouts: the conversation layout and the thread arc layout.
Here's a screenshot:
@masnick Just note that it can be a bit fragile. Not all view-combinations work well. And it might break on updates.— MailMate (@mailmateapp) November 22, 2013
Here's the code: