To reproduce:
1. Open any project
2. Create canvas with Screen Space - Overlay Render mode
3. Create multiple Game views of different resolutions
4. Try focusing different Game views

Expected: canvas scales according to the Game view focused
Actual: canvas doesn't scale on focus

Reproduced in: 2017.1.5f1, 2017.2.3p3, 2017.4.8f1, 2018.1.9f1, 2018.2.2f1
Not reproduced in: 2018.3.0a1, 2018.3.0a6

Note: canvas doesn't seem to have had on focus listener, therefore would only redraw if selected Game view is resized, or canvas properties are changed