Opera Mini on Android using high data savings (this means it internally uses the stock browser engine Webkit). I'll try changing both the theme and the browser and comment on the results. Edit: changing theme doesn't help. Edit 2: I tried on Chrome, problem persists. I have also noticed that the login page is not mobile friendly, the logo looks huge and the text is very small, I have to zoom in to write my username and password.
Can confirm login screen issue on Firefox for Android, but for the Shared by box I have different behaviour: Text size is consistent but it extends past the screen and can not be scrolled to. Forcible override of zoom in settings does allow for scrolling, though the box still extends past the "edge of the page". Chrome for Android, both issues present. Forcing zoom override makes the site just.. weird. Starts off zoomed too far in but does not seem to make the shared by box smaller. Another issue: In the user feed page, comments and post titles do not word-wrap on word boundaries, making it hard to read. Tested several themes and seemed present on each.
Thank you for testing it and also for discovering more things I overlooked.