When working with Microthemer, when I move my mouse towards the top left quarter of the page the mobile device sizing dropdown appears. It happens so often that it’s getting annoying. The trigger seems oversensitive in some way.
Thanks for sharing your niggles with accidentally triggering the device preview feature. I’ll definitely do something about that. The feature could just as easily be accessed with a click rather than hover. I’ve added this to my todo list and will try to get it sorted pretty soon. In the meantime, while you’re not needing to make use of the rulers, you can switch them off via View > Rulers.