To reproduce:
1. Open attached project ("UI_Mask_Issue")
2. Make Image B a child of an Image A
3. Drag Image B over Image A in the Scene window
-- Inspect the masking at the shadowed corner of the Image A

Expected: Image A is masked only on it's surface, shadow area excluded
Actual: Image A is masked both on it's surface and shadow area (see user's attached screenshot)

Reproduced with: 2017.4.18f1, 2018.3.1f1, 2019.1.0a13

Note:
- The issue reproduces with all UI elements affected by Shadow Component