How to get core sources for Maven based platform app

Hello,

(this is one of those embarrassing questions which I feel I should be able
to figure out - given that I've now worked with platform apps for several
years - but I really don't get it)

Here we go:
When working with a NB Platform app I often find myself debugging deep into
platform code.

For my direct deps, e.g. org-openide-util, this works fine as I can get the
IDE to download the sources from the NB Maven repo.  However for things
that are more in the core of the platform I wouldn't know how to get the
IDE to download the sources for me.

Consider this:  I'm happily debugging some code that uses WindowManager.
This source is available to me (via Maven repo) as it sits in
org-openide-windows. However I can only get so far as the actual Window
Manager, WindowManagerImpl, lives in core (org-netbeans-core-windows) and
for that I don't have the sources and wouldn't know how to get the IDE to
fetch it for me.

On my app module I can see the core of the platform as a bunch of ".nbm"
dependencies listed under "Non-classpath Dependencies". But when I
right-click one of them, say org-netbeans-core-windows, and select
"Download sources" then nothing happens.

How to solve this ?

Peter