Elements of a UI Canvas will resize themselves when entering play mode. This seems to be related to having non-0 offsets/anchors, and only happens in Canvases set to scale with screen size or have a constant physical size (constant pixel sizes are unaffected).

Reproduction Steps:
1. Open the attached project
2. Enter play mode
3. Observe all the UI moving
4. Leave play mode, observe that the new offsets/positions are kept
5. Repeat steps 2 and 3, observe things moving continually further away

Reproducible in versions 2017.3.0a1, 2017.3.0a3, 2017.3.0a6.
Not reproducible in version 2017.2.0b9.