When viewing a thread on the mobile browser, there is no longer a link to easily go back to the forum that contains the thread currently being viewed. The only exception to this is if you’re at the top of a thread. From anywhere else you have to scroll all the way back up, go back to the forum root, or use the dropdown selection.
Below is a screenshot. You can see that there’s no readily available way to go back to the current forum. For reference, this used to exist in the earlier days of the new forum.
Http://imgur.com/a/kIJO6LG