Long text is not scrolling inside the input

### Describe the bug

When the textarea contains enough content to overflow, scrolling with the mouse wheel/trackpad does not scroll inside the textarea. Instead, the outer chat view/page scrolls, which makes editing longer prompts difficult.

### Affected version

1.0.63

### Steps to reproduce the behavior

1. Open GitHub Copilot CLI chat.
2. Type or paste enough text into the textarea so that the input area becomes scrollable.
3. Move the cursor over the textarea.
4. Try to scroll with the mouse wheel or trackpad.

### Expected behavior

The textarea should capture the scroll event and scroll its own content while the pointer is focused/hovering over it.

### Additional context

OS: MacOS 15.7.7
Terminal emulator: Terminal
Shell: zsh

Reproduce the issue in GitHub Copilot CLI chat using a long textarea value, then locate the chat input and its scroll or wheel-event handling. Verify the behavior on the reported macOS Terminal and zsh setup; done means wheel or trackpad input over an overflowing textarea scrolls that textarea rather than the outer chat view.

索引モデルが issue の本文から書いたものです。