Two ways to improve the viewport ruler
1) have a toggle switch to leave the ruler on even once we let go of the viewport adjuster, so that we can see what device size we are currently on at all times.
2) add a vertical ruler that would give us the same functionality as the horizontal
This would make it way easier to create "outside of the box" design solutions without a lot of back and forth guess work to get the the design adapted to all devices. You would be able to see the exact ratio and size for each device.
If this was combined with custom breakpoints we would easily be able to design for different devices with minimal testing.