fix(terminal): close stale classic completion menu after backspacing past original prefix

[Jason-Shen2]

Summary

With Classic Completions enabled, pressing Tab to open the completion menu and then backspacing past the original prefix left the stale suggestion list on screen. The result set was computed for a different query than what was in the input, and backspacing to an empty prefix re-showed the entire original result set.

Root Cause

The TODO in update_tab_completion_menu (app/src/terminal/input.rs:11646) called this out explicitly: the staleness check that closes the menu when text_up_to_cursor no longer starts with the original buffer text was gated by && !self.is_classic_completions_enabled(ctx), so classic completions never invalidated stale results.

This exception was added because cycling through fuzzy matches could temporarily put non-prefix text in the buffer. However, the active-cycling case was already handled by a separate check (current_selected_item == current_word), which came after the staleness check and was therefore unreachable when the buffer diverged from the original prefix.

Fix

Restructure update_tab_completion_menu to:

1. Compute current_word early.
2. Check the active-cycling case before the staleness check so Tab cycling keeps the menu open in classic mode.
3. Run the staleness check in both classic and non-classic modes, removing the exception that prevented stale-result closure.
4. Flatten the control flow (no more if/else nesting).

This preserves Tab cycling behavior while properly closing the menu when the user edits away from the original prefix, matching the non-classic completions behavior.

Fixes #13605

Test plan

• Enabled classic completions mode.
• In a directory with Documents/ and Downloads/, typed cd Do → Tab → Backspace repeatedly.
• Verified that backspacing past the original Do prefix closes the stale completion menu.
• Verified that Tab cycling between Documents/ and Downloads/ still works correctly.
• Verified that continued typing after Tab (e.g. cd Doc) still filters results correctly.

CHANGELOG-BUG-FIX: Classic completions menu now properly closes when backspacing past the original query prefix instead of showing stale results.

[oz-for-oss]

@Jason-Shen2

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[oz-for-oss]

Overview

This PR updates classic tab completion menu invalidation so stale completion results close after the input no longer starts with the original completion prefix, while preserving active Tab cycling when the selected item matches the current word. I did not find code correctness, security, or spec-drift issues in the attached diff.

Concerns

• For this user-facing completion-menu behavior change, please include screenshots or a short screen recording demonstrating it working end to end. The PR description includes manual test steps, but repository review guidance requires visual evidence for user-visible behavior changes.

Verdict

Found: 0 critical, 1 important, 0 suggestions

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