Add tolerance for diagonal mouse movement from sidebar section title to flyout menu

Problem to solve

In the sidebar of our old navigation, it was possible to hover from a section title diagonally to the bottom nav item inside a flyout menu, with the mouse cursor crossing otther section titles, without the flyout closing and the next section's flyout to appear. This "diagonal tolerance" was done with a bit of jQuery code that will be somewhat tricky to replicate with our Vue components of the new nav.

See !124863 (comment 1449966208) for more details and discussion.