Steps to reproduce:
1. Open the attached project "case_1217253.zip"
2. Select "AnimationController" in the Project window
3. Open the Animator window (Window > Animation > Animator)
4. Select on the "BugTree" node and double-click on the "Blend Tree" Motion field
5. Click on either the left or right value under the Blend curve
6. Input any value and press enter
7. Click and hold in any Editor window and move your mouse

Expected result: Selected Editor window is focused and no Threshold values are changed
Actual result: Previously selected and edited value field can be changed by moving the mouse cursor

Reproducible in: 2017.4.36f1, 2018.4.17f1 , 20192.2.20f1, 2019.3.1f1, 2020.1.0a22

Notes:
- Enter key must be pressed for this to work. Simply selecting the field or clicking out of it when the value is changed does not reproduce the issue