See T13244. On devices, we don't have a left-hand navigation menu; it moves to the upper right in a "hamburger" menu ("≡") instead.
This currently isn't automatic, and it's easy to get wrong. D20093 is an example of correcting a mistake with specifying the menu behavior.
It's currently difficult to automatically get the behavior right in the general case because some pages have a hamburger menu but do not have a left-side navigation menu. An example is that Slowvote detail pages (`/V123`) have a hamburger menu with navigation links for the sort of parent context, but these links do not appear on the page on desktop.
In cases where we have a left-hand menu, I think the default behavior should pretty clearly be to show the same menu in the hamburger menu.
Barring some sort of epiphany about this, I'm currently inclined to remove the hamburger menu from other pages (pages where desktop has no left-hand navigation). On desktop, you just use the breadcrumbs to go back up; I think this is broadly pretty reasonable on mobile too?