How to reproduce:
1. Open the attached project ("maxsizeissue.zip")
2. Go to "QuickTool" -> "Open"
3. Observe that the window is not scalable (This is expected, the minSize and maxSize values are set to the same Vector2 value)
4. Press "My Button"
5. Try to scale the window (The maxSize is set to a slightly higher value while the minSize remains the same)

Expected result: the window is now scalable but only to a certain, small, set maxSize
Actual result: the window is now scalable but the max size is unlimited

Reproduced in: 2019.3.0a9, 2019.2.0b10, 2019.2.0a11
Not reproducible in: 2019.2.0a10, 2019.1.11f1
Couldn't test with: 2018.4.4f1