How to reproduce:
1. Create a new project
2. In the Hierarchy window, select "Main Camera" (a Camera Preview window in Scene view will appear)
3. In Scene view, enable "Tools" button
4. Shrink Game view and observe Camera Preview in Scene view

Expected result: Camera Preview aspect ratio changes even when the Game window is slim in width
Actual result: Camera Preview has a minimum width due to Tools panel for that reason Camera Preview does not represent the Game view properly.

Reproducible with: 2019.1.0b3, 2019.2.0a4
Could not test with: 2019.1.0b2 and older due to lack of feature - "Tools" panel/button