if you want to have the classical "diagonal" threads with full functionality, you probably should also put that functionality also to the new compact indendation.

what are currently missing in the flow's compact comments are : 1) several sub-threads to/from every post/comment/reply .

i have made an image to show that functionality in compact comments. that image also shows idea to put threads one behind another. It is here : https://www.mediawiki.org/wiki/File:Compact-comments.png . and i upload it also here.

fix : i delete "2) subthread for the latest post" - it is not feature of classic diagonal thread comments. so, conversely, it should not be used in "compact thread", to be convertable to "diagonal threads".

see edition to my previous post. and:

the classic "diagonal thread" comment system/interface can be "hacked" by users, if they are in agreement, - and, the "several subthreads" can be used as a thread, and "reply" can be used as "create a subthread", but, if they do so, then that system has the "subthread for the latest post" but lacks "several subthreads".

the classic threads system really does not have "subthreads", it has multiple continuations of threads, and second continuation is adopted/interpreted as a subthread in the flow's new indendation model, and third continuation is not possible there in flow.

i have made an image of classic comments modified into compact form, and with multiple continuations put one behind another. see: https://www.mediawiki.org/wiki/File:Compact-classic-comment-threads.gif