WebContentsView background doesn't cover existing views until pageload finishes

### Preflight Checklist

- [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/main/CONTRIBUTING.md) for this project.
- [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [x] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a bug report that matches the one I want to file, without success.

### Electron Version

37.0.0-beta.2

### What operating system(s) are you using?

macOS

### Operating System Version

macOS Sequoia 15.5

### What arch are you using?

arm64 (including Apple Silicon)

### Last Known Working Electron version

35.0.0-alpha.2

### Expected Behavior

When a WebContentsView with a non-transparent background color is added to a window, that background should cover existing child views that are earlier in the stacking order regardless of whether the new view has finished loading.

https://github.com/user-attachments/assets/20c8d8c3-4b94-4bad-9d49-3c5757d79793

### Actual Behavior

The background color of the new view doesn't render on top of existing views until it finishes loading. If no other child view is present the background is visible immediately; otherwise, the existing child view covers it until the load finishes. There's also a brief flicker of the background before the existing view covers it.

https://github.com/user-attachments/assets/60d642c9-1be9-434b-a566-d124f602f0c9

### Testcase Gist URL

https://gist.github.com/rf-figma/9ed0a7c5130d0ad91a851b5fad03eeed

### Additional Information

This works as expected in 35.0.0-alpha.2 and fails in 35.0.0-alpha.3 and all newer versions I tried, including the latest beta. I would guess that it's caused by #45238 (the backport of #44628).