How to reproduce:
1. Create a new project or open an existing one
2. Open the Game window
3. Resize the Editor to the smallest possible size
4. Enter Play mode
5. Try to resize the editor to an even smaller size

Expected result: It is not possible to resize the Editor to a smaller size than before
Actual result: It is possible to resize the Editor to a smaller size than before

Reproduced with: 2017.4.30f1, 2018.4.4f1, 2019.1.10f1, 2019.2.0b9, 2019.3.0a9

Notes:
After doing the reproduction steps the minimal window size will stay the same even after exiting Play mode.
The minimal window size will reset after the project is reopened.
There must be a Game window present in the Editor to reproduce this issue.