fix(tui): prevent underflow in dropdown navigation and scroll calculations

[echobt]

Summary

Fixes #5182, #5180, #5176 - Dropdown underflow and panic issues.

Problems

1. Division by zero when max_visible=0
2. Subtraction underflow in scroll offset
3. Index panic in visible_items

Solution

Added guards for zero values and used saturating arithmetic.

Changes

• dropdown.rs: Added guard for max_visible=0 in select_next/select_prev
• scroll.rs: Added guard for visible=0 and used saturating_sub in ensure_visible
• scrollable_dropdown.rs:
  • Fixed visible_items() to handle edge cases safely with .get()
  • Added max_visible=0 guard in select_next/select_prev/calculate_scroll_offset
  • Used saturating_sub to prevent underflow

[greptile-apps]

Greptile Overview

Greptile Summary

This PR fixes critical underflow and division-by-zero issues in TUI dropdown navigation (fixes #5182, #5180, #5176) along with several other robustness improvements across the codebase.

Key Changes

TUI Dropdown Fixes (Core Issue)

• Added max_visible == 0 guards in dropdown.rs, preventing modulo division by zero in navigation
• Added visible == 0 guard in scroll.rs ensure_visible() to prevent underflow
• Used nested saturating_sub operations: index.saturating_sub(visible.saturating_sub(1)) to safely handle edge cases
• Fixed visible_items() in scrollable_dropdown.rs to use .get() for safe slicing instead of panicking index operations

Additional Robustness Improvements

• Path Validation: Fixed validate_path_safety to check path components instead of substring matching, preventing false positives on filenames like file..txt
• Command Validation: Added normalize_command to prevent validation bypass via whitespace, quotes, or path variants (e.g., /bin/rm vs rm)
• Config Validation: Added custom deserializers for threshold_percent and sampling_ratio to enforce 0.0-1.0 range at deserialization time
• Race Condition Fixes: Fixed TOCTOU races in MCP server initialization and plugin registration using atomic check-and-set patterns
• Tool Response Storage: New bounded storage with capacity limits, TTL, and automatic cleanup (fixes #5292, #5293)
• Session Storage: Enhanced error handling to continue on errors while logging issues
• Type Safety: Changed token count fields from i64 to u64 throughout the codebase

Confidence Score: 5/5

• This PR is safe to merge - all fixes use defensive programming patterns that prevent crashes without changing behavior
• All changes are defensive improvements using saturating arithmetic, early returns, and safe slicing. The fixes directly address panic conditions with minimal code changes and include comprehensive test coverage
• No files require special attention - all changes follow consistent defensive patterns

Important Files Changed

Filename	Overview
src/cortex-tui-components/src/dropdown.rs	Added max_visible == 0 guard in select_next/select_prev to prevent division by zero
src/cortex-tui-components/src/scroll.rs	Added visible == 0 guard and nested saturating_sub in ensure_visible to prevent underflow
src/cortex-tui/src/widgets/scrollable_dropdown.rs	Fixed visible_items() with .get(), added max_visible == 0 guards, and used nested saturating_sub throughout
src/cortex-engine/src/tools/response_store.rs	New bounded storage for tool responses with TTL, capacity limits, and automatic cleanup (fixes #5292, #5293)
src/cortex-engine/src/validation.rs	Added normalize_command function to prevent validation bypass via whitespace, quotes, or path variants
src/cortex-mcp-server/src/server.rs	Fixed TOCTOU race in handle_initialize by holding write lock during check-and-set
src/cortex-plugins/src/registry.rs	Fixed TOCTOU race in plugin registration using HashMap entry API for atomic check-and-insert
src/cortex-tui/src/session/manager.rs	Enhanced error handling in add_tokens, pop_last_exchange, and add_message_internal with rollback on failure

Sequence Diagram

sequenceDiagram
    participant User
    participant TUI as TUI Component
    participant Dropdown as Dropdown/Scroll
    participant State as State Manager
    
    Note over User,State: Dropdown Navigation Flow (Fixed)
    User->>TUI: Press Up/Down Key
    TUI->>Dropdown: select_next() / select_prev()
    Dropdown->>Dropdown: Check max_visible == 0 guard
    alt max_visible is 0
        Dropdown-->>TUI: Return early (no crash)
    else max_visible > 0
        Dropdown->>Dropdown: Calculate new selected index
        Dropdown->>State: ensure_visible(index)
        State->>State: Check visible == 0 guard
        alt visible is 0
            State-->>Dropdown: Return early (no crash)
        else visible > 0
            State->>State: Calculate offset with saturating_sub
            Note over State: offset = index.saturating_sub(visible.saturating_sub(1))
            State-->>Dropdown: Offset updated safely
        end
        Dropdown-->>TUI: Selection updated
    end
    TUI->>User: UI updates without panic

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[greptile-apps]

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[echobt]

Consolidated into #69 - fix(tui): consolidated TUI fixes for overflow, positioning, and panic prevention