If I change the size of the Editor window / Scene panel, the Canvas changes size. Fixable?

I have a project that contains a Canvas and, belonging to that, a background (Sprite Renderer) that is almost as big as the Canvas. There is other stuff and it’s a 2D project. Unity 2022.1.12.

The Canvas component Render Mode is “Screen Space - Overlay”.

If I change the size of the Editor window or the Scene panel, the background moves off a long way from the Canvas and the Canvas changes size.

I can manually bring the background back to where it should be, but I cannot edit the Canvas Width and Height.

I can change the Render Mode to “World Space” and edit Width and Height but it does not have any effect on size of Canvas. Moreover, switching back to “Screen Space - Overlay” means the background moves off again and the Width and Height revert to the wrong values.

How can I get my Canvas size back to the way I first specified it?

Am I supposed not to resize my windows and panels? Or am I doing something wrong?

Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc:

Thanks for those. I’m doing some research.