How to reproduce:
1. Open given Unity project
2. Enter 'Main' scene
3. Enter Play Mode
4. Move panel around
5. Press right-bottom box to resize it and try to resize it

Expected result: Panel should be resized and kept in the same position
Actual result: Panel is resized but moved back to the previous position

Regression first introduced in: 2017.3.1p1
Reproducible with: 2018.2.0a5, 2018.1.0b12, 2017.4.0f1, 2017.3.2f1,
Not reproducible with: 2017.2.2p2, 2017.1.3p2, 5.6.5p2

Note:
Script is taken from this video https://unity3d.com/learn/tutorials/modules/intermediate/live-training-archive/panels-panes-windows?playlist=17111