Repro steps:
1. Create a particle system in 2017.3 (tried in a7, should work in any version after the curve Optimize button was introduced)
2. Select any curve and try to move the left key to the right.
3. Observe how the drag is interrupted by the appearance of the Optimize button.

Notes:
- Regression from 2017.2
- the same interruption happens when drag-selecting multiple keys
- gif attached