Please remove the left column where categories are listed visually and just have the drop down categories shown up above forum messages.
Then add category to the forum message itself that is listed on the home page, as does Lines and Orthogonal Devices.
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You can click your user icon in the upper-right corner, then click the gear, then click “Interface” in the left nav, then change “Default Home Page” to “Latest”. This achieves what you want. Works in any DIscourse forum.
Funny…we appear to be on all the same forums.
I have set the layout exactly as on lines, OD, Axoloti and mutable forums, just as @jasonw22 said. Btw, hello 
noted all above, and ha truu! happy to see some Line’s folks here as well.
Imagining a nice near-future with much less Facebook and more Discourse forums