Infinite zooming out is possible by mouse scroll which makes Scene grid, game objects, and gizmos disappear.

Steps to repro:

1. Create a new Project.
2. Zoom out in scene view using mouse scroll wheel.

Actual Result:
Zooming out in scene view is not restricted and Scene grid disappears when zooming out with the scroll wheel.

Expected Result:
Zooming out in scene view should be restricted.

Occurs on:
2019.1.0a9

Working Fine on:
2019.1.0a8, 2018.3.0b11

Environment:
Windows10, Windows 7 and Mac 10.13.