Regression: Arrow Up/Down no longer moves cursor to line start/end at boundary lines in input editor

Regression: Arrow Up/Down no longer moves cursor to line start/end at boundary lines

Description

In the Claude Code input editor, when the cursor is on the first line but NOT at position 0, pressing ↑ (Up arrow) should move the cursor to the beginning of the line. This no longer works — pressing ↑ does nothing when already on the first line.

Similarly, when the cursor is on the last line but NOT at the end, pressing ↓ (Down arrow) should move the cursor to the end of the line. This also no longer works.

This is a regression — this behavior worked correctly approximately 1-2 weeks ago in an earlier version.

Steps to Reproduce

1. Open Claude Code
2. Type a multi-line input in the prompt area
3. Place cursor on the first line, somewhere in the middle (not at position 0)
4. Press ↑ (Up arrow)
5. Expected: Cursor jumps to beginning of line (position 0)
6. Actual: Nothing happens

Same for last line + ↓:

1. Place cursor on the last line, somewhere in the middle (not at the end)
2. Press ↓ (Down arrow)
3. Expected: Cursor jumps to end of line
4. Actual: Nothing happens

Environment

• Claude Code version: 2.1.175
• Platform: macOS Darwin 25.3.0
• Shell: zsh
• Node: v22.22.1

Workaround

Ctrl+A / Ctrl+E (Emacs-style readline bindings) still work correctly to jump to line start/end.

Additional Context

This is likely a regression in the Ink/React-based TUI input component's arrow key handling at line boundaries. Standard text editor behavior (VS Code, most textareas) supports this navigation pattern.