Here's the UI as seen in Safari Technical Preview 41, released October 4, 2017.
Revised Responsive Design Mode
- Add a bandwidth throttling menu:
- Default Network
- High-Speed Cellular
- Low-Speed Cellular
- Improve the contrast of the selected viewport setting in a style consistent with the Web Inspector
- Show a rotate icon when hovering on a device icon
- Align label baselines
- Add a screen mode menu for iPads that would allow selection from:
- Full Screen
- Split View
This provides an explicit UI for using the iPad multi-tasking modes and would allow users to switch between portrait and landscape orientations of different iPad models while staying in a multi-tasking view. In STP 41, you can skip cycling through the iPad multi-tasking modes by Option-clicking the iPad icon.