Fullscreen UX fail on macOS

The top arender menu is hidden by window bar in fullscreen mode on macOS

How to reproduce:

• use the fullscreen action to make the arender iframe go fullscreen (assuming it is fixed with the solution proposed in https://jira.nuxeo.com/browse/NEV-665)
• try to display the top menu by moving the cursor to the top

=> the menu is displayed but hidden by the window top bar

There is only a very narrow zone where the cursor must be to display the menu without it being hidden by the window bar.