Reproduction Steps:
1. Import attached project "893585 - DropDown.zip".
2. Open "DropDownTest" scene.
3. Select Game View.
4. Make sure aspect ratio is set to custom Fixed Resolution 1920x1080.
5. Enter Play mode.
6. Select "Native - as a child" dropdown.

Expected result: Dropdown gives out list.
Actual result: Dropdown gives out empty list and freezes Game View. (GIF attached)

Reproducible with: 5.6.0f1, 2017.1.0a4
Not reproducible with: 5.5.2p3
Regression introduced in: 5.6.0b2

Note: On specific aspect ratios, different results occur. On some Fixed Resolutions, the parent Dropdown or both Dropdowns will freeze. Here is a list of all aspect ratios and which results occur on them:

Neither dropdown freezes:
16:9
1600x900
5:3
1280x768
800x480
8:5
1440x900
3:2
1280x854
4:3
2048x1536
1600x1200
1440x1080
1400x1050
1152x864
1024x768
800x600
768x576
5:4
2560x2048
1280x1024

Child freezes:
21:9
2560x1080
17:9
4096x2160
2048x1080
16:9
3840x2160
2560x1440
1920x1080
1366x768
1280x720
1024x576
854x480
8:5
2560x1600
1920x1200
1680x1050
1280x800
320x200

Parent freezes:
When dimensions are identical. (Doesn't always reproduce)

Both freeze:
3:2
1440x960
1152x768
480x320
4:3
1280x960
640x480