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[Damien Cassou]

> OK, I see with counsel-imenu the current indexing by header lines is
> reasonable. It might be improvable by adding the subject, but I'm
> not sure about line lengths.
> - maybe the docstrings should recomment counsel-imenu?

I'm not sure as the function
`notmuch-show-imenu-extract-index-name-function` is private and there
are other imenu frontends available. What about a NEWS entry instead
along those lines:

    * Add Emacs' imenu support in notmuch-show and notmuch-search

    Emacs' major modes can facilitate navigation in their buffers by
    supporting Imenu. In such major modes, launching Imenu (M-x imenu)
    makes Emacs display a list of items (e.g., function definitions in
    a code buffer). Selecting an item from this list moves point to
    this item.

    This release adds Imenu support to both notmuch-show and
    notmuch-search buffers:

       * in notmuch-show, Imenu will present a list of all messages in
         the currently visible thread;

       * in notmuch-search, Imenu will present a list of all messages in the
         search buffer.

    We recommand an external imenu frontend, such as counsel-imenu,
    which will make the experience much better that the default `M-x
    imenu`.

> I think the indentation should probably go to make it more usable
> with the builtin imenu

I did that in the patch even though I liked it with indentation better.