steps to reproduce
open main menue
press back button on your phone

results
back button on phone does not lead to exit the menue but to exit the page "behinde" the menue.

expected results
Since the main menue is displayed in fullscreen one could asume to have obend a new page by pressing the menue button and therefor try to exit this "page" via the back button on one's phone:

possible solutions:

1. make it possible to exit this menue via back button

or.

2. Display the main menue not in full screen but similar to the email action menue: