Screens for iOS 4.9.12 or later adds pointing devices support introduced in iPadOS 13.4. You can use a compatible mouse, trackpad or Magic Keyboard (to be released by Apple in May 2020) to control the remote computer Screens is connected to.
- Dragging (left-click and start dragging)
Please note that scrolling requires Magic Mouse 2, Magic Trackpad 2 or Magic Keyboard.
Unfortunately 3-finger gestures on Magic Trackpad 2 and Magic Keyboard are reserved by iPadOS but you can still trigger those via the device display. You'll find more details about other supported gestures in this article.
For more details about iPadOS and pointing device support, visit these articles from Apple: