/diff is hiding some lines seemingly at random

### Describe the bug

I noticed _sometimes_ the diff is not showing all lines:

The line 193 is not showing! The line "appears" (it's still looks like it's _under_ the "original" line) when I move my cursor to the missing line:

Git diff shows this code section just fine, I also can't see any weird characters in hexedit:
```c#
internal static JniRuntime.JniValueManager CreateValueManager ()
{
if (RuntimeFeature.IsMonoRuntime) {
return new AndroidValueManager ();
}

if (RuntimeFeature.IsCoreClrRuntime || RuntimeFeature.IsNativeAotRuntime) {
return new JavaMarshalValueManager ();
}

throw new NotSupportedException ("Internal error: unknown runtime not supported");
}
```

I noticed that different lines disappear as I scroll. As I scroll, the line 193 appears, but now line 196 is hidden:

### Affected version

GitHub Copilot CLI 1.0.42-0.

### Steps to reproduce the behavior

The bug appears seemingly at random, I don't know how to reliably reproduce it.

### Expected behavior

All lines are shown in the diff.

### Additional context

- macOS
- ghostty

從 /diff 入口開始，在 macOS 的 Ghostty 中捲動時重現缺少行的行為。將呈現的 diff 與 git diff 的輸出進行比較；當每一行 diff 在捲動過程中都保持可見且穩定時，即表示完成。

由索引模型根據 Issue 內容生成。