Steps to reproduce:

1) Open Unity and create a new project
2) Player Settings >> Resolution and Presentation >> make sure that Default Orientation is set to Auto Rotation
3) Build and Run project on a device
4) Rotate device to change screen orientation

Expected result: There should be black borders on the screen edges between transition from landscape to portrait and vice versa (check attached video 'expected.mp4')
Actual result: White borders appears on the screen edges between transition from landscape to portrait and vice versa (check attached video 'actual.mp4')

Reproduced with:
5.6.0b3, 5.6.1p1, 2017.1.0b7, 2017.2.0a1

Not reproduced with:
5.5.3p4, 5.6.0b2

Regression since:
5.6.0b3

Devices under test:
Reproduced with:
Samsung SM G950F, OS:7.0, CPU:arm64-v8a, GPU:Mali-G71, Build:samsung/dreamltexx/dreamlte:7.0/NRD90M/G950FXXU1AQDG:user/release-keys

Samsung SM G950U, OS:7.0, CPU:arm64-v8a, GPU:Adreno (TM) 540, Build:samsung/dreamqltesq/dreamqltesq:7.0/NRD90M/G950USQU1AQC8:user/release-keys

Not reproduced with:
HUAWEI EVA L19, OS:7.0, CPU:arm64-v8a, GPU:Mali-T880, Build:HUAWEI/EVA-L19/HWEVA:6.0/HUAWEIEVA-L19/C432B182:user/release-keys

Samsung SM G930F, OS:7.0, CPU:arm64-v8a, GPU:Mali-T880, Build:samsung/heroltexx/herolte:7.0/NRD90M/G930FXXU1ZPK3:user/release-keys

iPhone 7 Plus iOS 10.3.2

Note: white borders appears with all graphics API(GLES3, GLES2, Vulkan)

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Fixed in: 2017.2.0b3