How to add a new "display" to speedbar? (minor-major modes)

[Vladimir Nikishkin]

Hello, everyone

I am reading the speedbar manual, and I am a bit confused about it.

Firstly, a question for basic understanding:
a "minor" and a "major" mode for speedbar are not defined similarly to
the main Emacs' "minor" and "major" modes, right?
I.e., a "minor" mode does not "conservatively extend" a major mode?
Do I understand correctly?

If yes, then is it true that a better equivalent of an Emacs' "major"
mode is a speedbar's "display"? (Which can be somehow related to the
default-directory of an associated buffer, in which case it is a "major"
mode, or unrelated, in which case it is a "minor" mode?)

My question is how to add a new "display" to speedbar? The motivation is
really updating mu4e's mu4e-speedbar library for modern Emacs.
So far mu4e-speedbar has some code to generate a new speedbar buffer
when a context is changed, but the current "display" of speedbar is not
changed, which can be verified by "mouse-3 Speedbar Displays".

I want "mu4e quick links" to be available there, that would just call
the functions already available in mu4e-speedbar to generate the list of
bookmarks/maildirs.

-- 
Your sincerely,
Vladimir Nikishkin (MiEr, lockywolf)
(Laptop)