Horizontal scrollbar is not scrolling all diff editors

I think we should make all editors inside a multi-diff editor forcefully have the same width such that they can scroll, the current scrolling seems incorrect to me, it breaks the illusion that this is one widget I'm operating.

https://github.com/user-attachments/assets/a45c976e-911d-498b-8e62-d282cc88bb0f

```
Version: 1.139.0-insider
Commit: c74ba73b780a4a33173c006e52560d61432f53d6
Date: 2026-09-16T04:43:54Z
Electron: 42.10.0
ElectronBuildId: 15109253
Chromium: 148.0.7778.280
Node.js: 24.18.1
V8: 14.8.178.38-electron.0
@github/copilot: 1.0.84-5
@github/copilot-sdk: 1.0.13
OS: Darwin arm64 25.6.0
```