How to reproduce:
1. Open the attached project '1149313repro'
2. In SampleScene, go to Particle System object and enable Size over Lifetime module
3. Click on the 'Size' curve and add a key somewhere in that curve
4. Alt-tab out and back in and try to move that key

Expected results: Key is moved to a new position
Actual results: Key is stuck

Reproducible with: 2017.4.26f1, 2018.3.14f1, 2019.1.0f2, 2019.2.0a2
Not reproducible with: 2017.4.26f1, 2018.3.0b4
Regression between 2018.3.0b4 and 2018.3.14f1, not finding the exact one because it is already fixed.

Note: The alt-tabbing doesn't have to take place when Particle System is selected. It seems that after one alt-tab, the keys will always be stuck.