@kas1e

The problem is, that workbench does not know of any gadgets other than its own.
It still is a plain intuition window (no reaction class or boopsi class).
So the rendering uses the whole window area.
Any gadgets you place inside the windows area will be painted over by the workbenchs rendering.

Therefor a solution is, to put the custom gadgets inside the window border.
Well, this means that the top border is problematic, because of the title.
If you increase the top border height, the title will be centered and gets in the way.

Hi dudes,
the limitation is an internal buffer of ContextMenus commodity. It only reserves 256 bytes, to pass to the CLI.
That's bad news.

I'm currently looking into it and hope to fix it soon.

Take a look at the autodocs: workbench.doc
The WorkbenchControl() function does just what you want.
The tag WBCTRLA_GetSelectedIconList will give you a list of all selected icons for a given drawer (=WB window).
The WBCTRLA_GetOpenDrawerList gives you a list of all open drawers. So you can loop through them and call WBCTRLA_GetSelectedIconList for each one.

This information is not in the Wiki yet. Also the multiple hooks, you can use via ChangeWorkbenchSelection() are not yet described in the Wiki.
Same goes for icon.library, which has got some interesting methods, since 3.5.