@Jdlrobson there is no connection between T216152: AMC Navigation - add new links to main menu with click tracking and this task. The T216152 task is only backend work that allows MainMenu to be easily modified/extended, this task is clearly a frontend work, that adds scrolling (js/css/html).
Also T216152 is mostly done, in Ready For Signoff now.

It's ready to work on

I think this could be as simple as adding the following rules:

.navigation-enabled {
  overflow-y: hidden;
}

#mw-mf-page-left {
  overflow-y: scroll;
  height: 100%
}

and removing the following rules:

.navigation-enabled #mw-mf-viewport {
  height: auto !important;
}

#mw-mf-page-left {
  min-height: 100%;
}

I think this could be as simple as adding the following rules:

I like this task and hope we do it and that it's simple. If it's not simple, it may not be worthwhile since the main menu doesn't have many items in it.

Change 526732 had a related patch set uploaded (by Niedzielski; owner: Stephen Niedzielski):
[mediawiki/skins/MinervaNeue@master] [UI] [main menu] make menu scrollable

https://gerrit.wikimedia.org/r/526732

Scrolling indicator — it would be great to communicate to the user that there are more menu items offscreen/blow. When the menu is first opened the scrollbar should appear then quickly fade out, indicating the additional contents below (it seems like sometimes browsers do this by default - need to investigate further)

I've submitted a simple patch that adds a scrollbar as needed but more work is needed for fancy fade in / out scrollbars. This remaining work is available if someone wants it. @Jdrewniak, are these practical to do in CSS or is JS necessary?

Change 526732 merged by jenkins-bot:
[mediawiki/skins/MinervaNeue@master] [UI] [main menu] make menu scrollable

https://gerrit.wikimedia.org/r/526732