A couple of weeks ago, I upgraded my mail client, Mu/Mu4e, to version 1.10.2 with Homebrew. It has some nice features but it also insisted on opening mu4e in full-frame mode, wiping out whatever window configuration I had. That was definitely not something I wanted. I did a quick scan of the documentation to see if there was some new configuration variable controlling this behavior but I didn’t find anything. Then I asked DuckDuckGo but either my search-foo was inadequate or there was nothing relevant.
This seemed like a breaking change to me so I thought maybe it was an error and perhaps a point release was available that fixed it. I checked with Brew and, sure enough, there was a 1.10.3 release so I upgraded. That didn’t help at all. In fact, Emacs wouldn’t even load. That was due to a library incompatibility and I had to recompile Emacs to get back to where I was.
My next plan was to run mu4e in a separate frame. That worked okay but made my Emacs navigation a little fussy. Finally, in desperation, I made another run at DuckDuckGo. Either my search-foo had magically improved or new information had materialized but I found this GitHub PR.
The TL;DR is that you can fix the problem by adding
(add-to-list 'display-buffer-alist
`(,(regexp-quote mu4e-main-buffer-name)
display-buffer-same-window))
to your configuration.
If, like me, you don’t want mu4e trampling your window configuration, just add the above to your init.el.