How to reproduce:
1. Open attached Unity project "rectmask2d-bug"
2. Select File -> Build Settings
3. Add "Scene" scene
4. Switch Platform to WebGL
5. Build and Run
6. Scroll or drag images with text

Expected result: Images stay in the range of the Scroll View Gameobject while scrolling (984147_Expected.gif).
Actual result: Images get out behind the edges of the Scroll View Gameobject while scrolling (984147_Actual.gif).

Workaround: Change z position to 0 of the "Bad Z 1 object", located in the Canvas -> Z Positioning Container -> GameObject.

Regression first introduced in: 2017.2.0b1 (8ea65b152036).
Reproducible with: 2018.0b2, 2017.3.0p1, 2017.2.1p2.
Not reproducible with: 2017.1.3f1, 5.6.5f, 5.5.6f1.

Note: This is working properly in the Standalone build and in the Editor.