Description: RectMask2D doesn't update when the parent size is changed. After increasing parent size the "Child" game object should be visible, but it's not.
You need to manually disable RectMask2D component on "Parent" game object, and then enable it again to make "Child" game object visible.

To reproduce:

1. Open the 'main.unity' scene.
2. Run the game.
3. Click on 'Enlarge parent'
This will increase parent size, but child object is not visible.
4. Click on 'Fix mask'
This disables and enables RectMask2D component, and then child game object becomes visible.

Note: You can do step 4 first and then step 3, child object will be visible as long as you disable and enable RectMask2D component at some point while in Play mode.

Reproduces with: 5.3.0f4: 5.4.3p3: 5.5.0p1; 5.6.0b1

Note: In 5.3.0f4 disabling and enabling the component doesn't fix the problem.