Margins for Panel widget?

With Window skins, there is a inner client widget which actually contains the child widgets... you can safely position children right to the edge of window->getClientWidget().

But panels don't seem to work this way, with standard PanelSkin if I put widgets right at the edge they actually overlap the panel border.

Is there a safe way in code to know where I can put widgets so they are properly 'inside' the Panel, or should I just leave 2px or something like that? Ideally, changing the skin to one with thicker borders wouldn't break my layout!

No, we don't have such things as margins. Easiest way would be to create child widget with empty skin and with coordinates of client area. And then create widgets on it.
Or adjust positions manually in editor.