Allow disabling chat-input edge-of-text history fallback (up/down to history when cursor at first/last line)

Problem

When up / down are unbound (null) in the Chat context via ~/.claude/keybindings.json, the input box correctly uses them for vertical cursor movement in the middle of a multi-line message — but the harness has a separate edge-of-text fallback that still routes the keystrokes to history:previous / history:next when the cursor is on the first line (up) or last line (down).

This fallback cannot be disabled through keybindings.json: setting the key to null removes the explicit action binding, but the edge-of-text behavior is independent code in the input widget itself.

Repro

1. Add to ~/.claude/keybindings.json:

``json
{
"bindings": [
{ "context": "Chat", "bindings": { "up": null, "down": null } }
]
}
``

1. Restart Claude Code.
2. In the input box, type a multi-line message (use ctrl+j for newlines).
3. Move the cursor to the first line and press up — message is replaced with the previous one in history.
4. Move the cursor to the last line and press down — message is replaced with an empty input ready for new text.

Impact

Losing typed content to a single misplaced arrow keystroke is a real pain point — users who type long multi-line prompts (which is increasingly common with agentic workflows) routinely hit the edge of the input and lose work. The current workarounds all have downsides:

• Binding up / down to a no-op action like app:redraw prevents the fallback but also kills cursor movement entirely.
• Moving history to a chord (e.g. cmd+up) doesn't help — the fallback still fires regardless of where history:previous is bound.
• The macOS-reserved modifier space is crowded, leaving few clean chord options for history.

Proposed solution

Add a setting (in ~/.claude/settings.json) such as chatInputEdgeHistoryFallback: false (default true for back-compat) that suppresses the edge-of-text history routing. When disabled, up at the first line and down at the last line do nothing (or, ideally, behave as Home/End — placing the cursor at line start/end). The current behavior remains the default.

Alternatively, treat unbinding history:previous / history:next (via null on up / down in Chat) as a signal to disable the fallback too — i.e., the fallback only fires when the action is reachable via some bound key in the active context.

Workaround in the meantime

None that does not sacrifice cursor movement.

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Filed via the Claude Code agent itself on behalf of a user who lost typed content to this fallback and is actively bothered by it.