How to reproduce:
1. Open the attached "1323621_Repro" project
2. Open the Scene "SampleScene"
3. In the Hierarchy select the "Particle System" GameObject
4. In the Inspector under the Particle System Component expand the "Emission" and select "Rate over Time" graph
5. If needed expand the Particle System Curves
6. Select the curve's endpoint and try to edit the curve's handle

Expected results: There's a way to zoom in or another way to move the curve so you're able to grab and adjust the handle
Actual results: There's no way to reach the handle without moving the endpoint

Reproducible with: 2018.4.33f1, 2019.4.23f1, 2020.3.2f1, 2021.1.1f1, 2021.2.0a10

Notes:
- It's possible to use CTRL+mouse wheel to zoom in and if the handle's grab point isn't too high it's possible to grab it, but if it's high enough even that won't help
- The issue is not present with other curves (like in the Animation window or variables from script) as those windows have infinite scrolls so you can move/zoom to always be able to grab the handle