On the Magento/Blank theme and themes inheriting from Magento/Blank the mobile menu is expanding too far off the screen when active.
The nav element .nav section has a width that is calculated as such:
width: calc(100% - 54px)
However the calculated width is near double the width of the html itself. This behavior is also only seen when viewed via responsive emulation (Chrome device mode for example) or on a mobile device. Simply reducing the size of the window will give a proper width.
Has anyone else ran into this same issue? Using the Magento/Luma theme does not have this issue. I cannot provide a url for the site as it is local to my machine and I was unable to find an M2 demo on the internet that is not using Magento/Luma.
Menu in device Mode:
Menu with reduced window size