TYPO3 Core

This is still a problem in 6.2.13.

With with special = directory or special = list, ACT and CUR is not working for mounted pages! (other special values not tested)
It works, when using entryLevel instead of special.

i think we have the same problem on T3 6.2. we have a HMENU with entrylevel 0 and a few mount points.
the menu works fine, but if you click on a link which is a mounted page in the tree, and you are there, then the url from the original page (the page you see, the page which is mounted) is wrong in the menu. it has the url from the mounted page you already clicked. i don't fully understand the behavior, but i think it goes in the same direction. mounted pages, hmenu and the active state.

Still not working in 6.2.15.
Since I don't expect this to be solved in the near future, I think a notice/warning in the Typo3-documentation is necessary. Users should be aware that HMENU won't work as expected when using mount points.

still alive in 7.6

I don't trust it yet, but it might be that config.MP_disableTypolinkClosestMPvalue = 1 helped...

It seems that helped!

What leads us to getClosestMPvalueForPage() in ContentObjectRenderer():

    /**
     * Returns the &MP variable value for a page id.
     * The function will do its best to find a MP value that will keep the page id inside the current Mount Point rootline if any.
     *
     * @param int $pageId page id
     * @param bool $raw If TRUE, the MPvalue is returned raw. Normally it is encoded as &MP=... variable
     * @return string MP value, prefixed with &MP= (depending on $raw)
     * @see typolink()
     */
    public function getClosestMPvalueForPage($pageId, $raw = false)

... what could be the use case for this method?

Nah. It's just broken. IMHO Mount Points should be removed.

And to not only rant, here's a quick & not-so-dirty solution for (very) simple mountpoints that can be configured via TS per page.

Supposing all your content is being read out via styles.content.get, for example:

lib.h1 < styles.content.get
    lib.h1.select.where = colPos=1

and then used in a fluid template.

Make a TS file that adds to all those statements:

lib.h1.select.pidInList = {$pids.pidMountedPage}

Include this file and set the constant. Done. Of course this is not recursing and doesn't add the page to the menu, and editors can't handle it themselves. But it's much more convenient than to include referenced content per insert-records. Also, if editors add an item on the source page, it will automatically find it's way to the target page.