Improve offset computation to avoid scrollbar when possible

In https://nightly.nuxeo.com/nuxeo/ui/#!/browse/default-domain/workspaces/Start%20Creating%20Your%20Cont you can witness the scrollbar on the right hand side is not needed if the window is high enough.
However, whatever the window size, the scrollbar appears.

Request is that the CSS height can be overridden with no hardcoded value, so that customers can choose whether or not the scrollbar appears
( e.g. in https://github.com/nuxeo/nuxeo-web-ui/blob/release-10.10-HF13/elements/document/nuxeo-collapsible-document-page.html#L64 for hardcoded values)