Ctrl-D in  ask_user /elicitation form fields triggers session shutdown and discards in-progress input #4866

Describe the bug

Summary: In the main prompt composer, Ctrl-D behaves correctly (deletes the character under the cursor when the line is non-empty; only signals EOF/shutdown on an empty line — the behavior fixed some months ago). However, when the agent presents an  ask_user  elicitation form and I'm typing an answer into a form text field, pressing Ctrl-D immediately triggers "shutdown" and discards everything I'd typed into the form. It appears the non-empty-line guard applied to the main composer's Ctrl-D handler was never applied to the elicitation/form input widget.

Impact: Repeated loss of carefully composed answers (happened ~6 times in one session).

Severity: High — silent data loss of user input; unexpected shutdown.

Suggested fix: Route the form field's Ctrl-D through the same handler/guard as the main composer (non-empty line ⇒ delete-char; empty ⇒ ignore or blur, not global shutdown).

Affected version

Copilot CLI 1.0.83, macOS (darwin-arm64)

Steps to reproduce the behavior

Steps to reproduce:

1. Have the agent call a tool that shows an  ask_user  form with a free-text field.
2. Start typing a multi-line/long answer into the field.
3. Press Ctrl-D (e.g. out of habit, to delete a character).
4. Observe: the session shuts down / the form is torn down and the typed answer is lost.

Expected behavior

Expected: In a form field, Ctrl-D should either delete the character under the cursor (matching the main composer) or be a no-op — it should NOT invoke global shutdown while a non-empty field has focus.

Actual: Ctrl-D invokes global shutdown regardless of field contents, discarding in-progress input.

Additional context

No response