Steps to reproduce:
1) Open the attached project.
2) Build for iOS.
3) Run on an iPad Air.
4) Push the colored button while in Portrait mode.

A dropdown list appears, however, only half of it is visible. The other half clips though the edge of the screen. This only happens when Canvas UI Scale Mode is set to 'Constant Physical Size' in Portrait mode. If the button is pressed in Landscape mode and the iPad is then rotated to Portrait, the dropdown is displayed as expected.

Reproduced on:
2018.1.0a3, 2017.2.0f3, 5.5.0b4

Devices tested:
iPad Air 2 iOS 9.2 - reproduced
iPhone 6S iOS 11.0 - did not reproduce
iPad Mini 2 iOS 10.3.1 - did not reproduce
Samsung Galaxy S7 Android 7.0 - did not reproduce