Reproduction steps:
1. Open project attached.
2. Open "test" scene
3. Open Game window
4. Change Game window size to aspect ratio, for example 16:10. (That includes Free Aspect).
5. Make sure there are gaps on left and right.
6. Move Game window.

Expected result: Nothing changes.
Actual result: Canvas sizes changes. It also impacts view in Game view.

This problem does not appear when:
1. There are gaps from top and bottoms sides.
2. When using fixed resolution.

Regression found in: 5.4
Works fine with: 5.3.7f1, 5.3.0f4, 5.2.0f3
Reproduced on: 5.4.0f3, 5.4.2f2, 5.4.3f1, 5.5.0f1, 5.6.0a3.