status-planned This is a temporary artifact due to responsive NOT being ready for all pages. So, on a smaller viewport (like a tablet) the left nav will auto hide on some pages (responsive ones) but not others (non-responsive ones). This will be fully addressed when we make all pages responsive, but that will take some time.
There is a curious inconsistency whereby on my tablet the left navigation shows on some pages (e.g. my user page) but not on others (e.g. the homepage).
User page: note the left navigation:
Home page: note the absence of left navigation:
This leaves the homepage apparently with less functionality than it had in the previous design. It turns out that it's still possible to e.g. list questions by criteria other than "Top", but it requires finding which of three "hamburger" icons (circled in screenshot above) opens the menu with that functionality, which I do not consider good discoverability.
I regard this as a removal of useful functionality: previously it took one click to get to a list of questions sorted by Newest, but now it takes at least two, and one of them is not discoverable.