I accidentally pressed some keys and found a feature that looked really useful. It gave me the option to hierarchically select a command/function to run in the minibuffer. There was some explanatory text at the top, saying I could use PageUp to go to the window to select completions. I hesitate to call it completions, because, while it was very similar to the window you get with
M-x ?, it wasn't that (for one, that text about PageUp isn't there).
With the latter you get an alphabetical list of commands. With the feature I stumbled upon, there was some hierarchy to it. E.g. there was an option "file". When I selected that with
f I got more options, like
I immediately installed
command-log-mode, but I haven't reached it again so far.
GNU Emacs 27.1 (build 1, x86_64-w64-mingw32) of 2020-08-21