My second monitor is set at 1280 x 1024 and I have noticed a bug in responsive mode. Because at this resolution upfront requires a horizontal scroll bar the subregions have a faulty offset that makes it impossible to move items to the right of it.
The region gets resized the smaller the window gets. This only happens for subregions it seems.
I have tested local, online and on the WPMUDev website.
Here is a video where you can see the corner tab of the region moving with the size of the window. It means anyone not on a widescreen display can't edit it properly.