I'm not sure if this is being discussed elsewhere - or if it is an issue at all. But, I thought I'd raise the issue.
In researching this issue- I set up Drupal 8 to view on my mobile device (iPhone - iOS6). As I navigated the admin interface (Seven), I realized that I was unable to view the breadcrumbs because they were blocked, along with much of the page, by the menu. When I selected a menu item, I expected the menu to go away and to see the page I had just selected. My first reaction, was that it was impossible to "hide" the admin menu and I began the process of reporting this as a bug.
Finally, I realized that I just needed to click on the menu item again, to close it. After feeling kinda stupid for a minute, I began to wonder if this might be a legitimate usability issue.
I'm not sure if this is standard behavior for this kind of mobile interface, because I don't actually do a lot of mobile web browsing myself. I do some. However, it seemed very counter-intuitive to me that selecting a menu item did not automatically close the menu.
The attached screen shot is using my browser, but I tested on my iPhone and got the same results.