/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

/diff エントリポイントから開始し、スクロール中に macOS の Ghostty で欠落行の挙動を再現します。レンダリングされた diff と git diff の出力を比較します。スクロール中もすべての diff 行が表示されたまま安定していれば完了です。

索引モデルが issue の本文から書いたものです。