Repro steps:
- Have Game View docked separately from Scene View and not being the front tab (see image)
- Create new scene.
- Menu: GameObject > Create UI > Button
- Frame select the Canvas so it's visible in the Scene View.
- Click on the Game view tab.
- Notice that the Canvas doesn't match the size / aspect ratio of the Game view.
- Resize the Game View.
- Observe the canvas size suddenly updating to the Game View size