The Upstream RTD theme has added some nice features for collapsing the navigation. You can see an example in pip Sphinx documentation. We have some users that could benifit from this, like GlusterFS, as they have many pages.

As a side, I happen to think that it is a negative update for smaller projects so I could be tempted to make this optional. Perhaps with an extra: config value like "extra: collapsable_nav: True" which would be specific to this theme? It could either remove the feature totally or expand by default.