How to reproduce:
1. Download attached project file ant open "Main" Scene
2. Enter Play Mode
3. Press "Button" in Game Window

Expected results: After the click, the button should have anchor mins set to 0, anchor maxes set to 1, and a top/left/bottom/right values all set to 0. Visually, it will appear as a square and fill the object labeled Destination
Actual results: The button is re-parented, however, it does not fill Destination, and instead gets a width of 160 and a height of 30

Reproduced with: All 2017.2, 2017.3.0b4
Not reproduced with: 5.6.4f1, 2017.1.2p1, 2017.3.0b5
Regression since: 2017.2.0a1 till 2017.3.0b5