Using mobile web on touch screen devices requires double tap for switching among inbox and achievements pages:
While inbox page is active, tapping on achievements icon close the inbox. And you need a second tap for open achievements page. Same behaviour for reverse switching. And for full version of site too (as I looked afterward).
If you open mobile web on the desktop (or another device with mouse pointer) there is no need to click for switching. Moving mouse to the appropriate icon is enough.
Tested on Android 6: Chrome 54.0.2840.85, Opera 37.0.2192.105088, Firefox 50.0.2