Keep the menu on top of the visualization in narrow windows
Allow to scroll the menu when its height overflows the page

The height of the menu is set approximately to 90% of the height
of the page. It should actually be set to 100% minus the top margin.
Use the same unit for the top margin and the height of the menu

Using percentages in both cases, rather than em units for the margin
and percentages for the height, should avoid discrepancies as the
height of the window changes.
Merge branch 'layout'