Animation clips extend from the left handle.

Steps to repro:
1. Open attached project > MultipleClips scene.
2. Click on Window > Timeline.
3. Select Cube gameobject from the Hierarchy.
4. In the Timeline window try to extend Animation clip from the left handle.
5. Animation clip is extending.

Expected result:
Animation clip should not extend from the left handle.

Reproducible on:
2018.1.0b2, 2018.1.0b1.

Works fine on:
2018.1.0a7, 2018.1.0a6, 2018.1.0a5, 2017.3.0f3.

Environment:
Occurring on both Windows & MAC.