Copying lines from a file or change only copies part of the selected lines

### Short summary

Copying lines from a file or change only copies part of the selected lines

### Affected version or release

v1.0.6

### Installation context

_No response_

### What happened?

When copying code or text from a file view or a change (diff) in the app, only part of the selected lines are copied to the clipboard. For example, selecting multiple contiguous lines and issuing Copy (Cmd/Ctrl+C or context menu) results in the pasted content containing only some of the selected lines or a truncated portion of the selection. This breaks workflows where users need to copy code from the app into editors, PR descriptions, or external tools.

### Steps to reproduce

1. Open the GitHub app and navigate to a repository.
2. Open a file or a change/diff view.
3. Select multiple lines across the file or change (click+drag or shift+click).
4. Press Cmd/Ctrl+C or use the context menu to copy.
5. Paste into an external editor or text field and observe that only part of the selected lines were copied or the content is truncated.

### Expected behavior

All selected lines should be copied in full to the clipboard so that pasting reproduces the exact selected text.

### Additional context

App version and OS not provided by reporter. Related or potentially-relevant existing issues: https://github.com/github/app/issues/999 (double-click/highlight behavior in code review UI) and https://github.com/github/app/issues/607 (copy buttons on Linux AppImage). Reported via TechWatching on 2026-06-24.

使用列出的选择和复制步骤，在文件视图和 diff 视图中重现该问题，然后定位现有的选择到剪贴板处理逻辑。当复制连续选中的行并将其粘贴到外部编辑器时能够保留完整选择，即表示完成。

由索引模型根据 Issue 内容生成。