OK, after playing around for a few minutes I found that scroll views (such as Table, Collection etc.) can have an offset automatically added to their top when "Adjust Scroll Views Insets" is checked in the Window's inspector. So they should be snapped to the top of the window.

This works really well, and if you're using a custom "plain" view for your main window content then unchecking the "Adjust Scroll Views Insets" gives it an offset, and any contained scroll views will just be positioned normally. <= I think there is a bug with this when the custom view is anchoring items to the left and top, need to investigate.

Neat, I think I rescind my feature request!

It would be great to have an alignment guide when moving controls around that snaps to the bottom of the currently showing Navigation or Status Bar area.

At the moment the only top-most "snap" is for the top of the window.

I've got to ask this ... it's all fancy and all, but why does 3D Preview exist?

Is it something left over from testing the framework that was just too cool that you couldn't bring yourselves to remove it? ;-)