[BUG] Cannot Move Cursor Up/Down Across Lines When Inside File Mention in Prompt Input

Environment

• Platform (select one):
• [x] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: <!-- output of claude --version --> 1.0.24 (Claude Code)
• Operating System: <!-- e.g. macOS 14.3, Windows 11, Ubuntu 22.04 --> macOS 15.4.1（24E263）
• Terminal: <!-- e.g. iTerm2, Terminal App --> terminal in Cursor IDE, Terminal App

Bug Description

<!-- A clear and concise description of the bug -->
When editing a prompt in the CLI, if a file mention (e.g., @path/to/file.py) is inserted in the middle of a multi-line input, the cursor navigation with arrow keys becomes unintuitive and cumbersome.
Specifically, if you move the cursor into the middle of a file mention using the arrow keys, you cannot easily move the cursor up or down to the previous/next line. Instead, the arrow keys only navigate through the file mention suggestion candidates below, and you have to use left/right keys to escape the mention and return to normal text navigation. This breaks the natural text editing flow.

Steps to Reproduce

1. Start editing a multi-line prompt in the Claude CLI that includes an actual file mention, such as:

hello world hello world hello world\
This @path/path/path/path/path/path/file\_name\_filename.py file's code,\
hello world hello world hello world

1. Move the cursor (with arrow keys) into the middle of the file mention (@path/path/path/path/path/path/file_name_filename.py).
2. Attempt to move the cursor up or down using the arrow keys.

Expected Behavior

• I expect to be able to freely move the cursor up or down to the previous or next line, even if the cursor is in the middle of a file mention.

Actual Behavior

• When the cursor is inside a file mention, pressing up or down arrow keys only moves the selection between file mention suggestions shown below the input area, rather than moving the cursor within the text.
• To move the cursor to another line, I have to use the left/right arrow keys to exit the mention first, which is cumbersome, especially when editing longer prompts with multiple file mentions.

https://github.com/user-attachments/assets/15d6c226-e3e8-4a2a-9203-dacb010638a8

Additional Context

<!-- Add any other context about the problem here, such as screenshots, logs, etc. -->
If a shortcut like Cmd(Ctrl) + Up/Down could be implemented to explicitly switch between "candidate navigation" and "text navigation", it would significantly improve usability for prompt editing.

Thank you!