Summary

Fix a Windows-specific bug where RUST_LOG environment variable (commonly set for tracing) causes tracing log messages to leak into the DeepSeek-TUI alt-screen, corrupting the terminal display.

Problem

When running DeepSeek-TUI with verbose logging enabled (DEEPSEEK_LOG_LEVEL=debug or --verbose), the env_requests_verbose_logging() function in crates/tui/src/logging.rs was checking both DEEPSEEK_LOG_LEVEL and RUST_LOG:

pub fn env_requests_verbose_logging() -> bool {
    std::env::var("DEEPSEEK_LOG_LEVEL")
        .ok()
        .is_some_and(|value| log_value_enables_verbose(&value))
        || std::env::var("RUST_LOG")  // ← causes issue on Windows
            .ok()
            .is_some_and(|value| log_value_enables_verbose(&value))
}

RUST_LOG controls the tracing subscriber filter in runtime_log.rs (file logging). On Windows, stderr is not redirected to the log file. When RUST_LOG is set to trace/debug/info, it incorrectly activates verbose CLI output, causing tracing messages to spill onto the TUI alt-screen display.

Fix

Remove RUST_LOG from env_requests_verbose_logging(). Only DEEPSEEK_LOG_LEVEL should gate CLI verbose output:

/// Note: RUST_LOG is intentionally NOT checked here — it controls the
/// tracing subscriber filter in runtime_log.rs (file logging) and
/// should not gate CLI verbose output. On Windows, where stderr is not
/// redirected to the log file, coupling the two causes tracing log
/// messages to leak into the TUI alt-screen.
pub fn env_requests_verbose_logging() -> bool {
    std::env::var("DEEPSEEK_LOG_LEVEL")
        .ok()
        .is_some_and(|value| log_value_enables_verbose(&value))
}

Steps to Reproduce

1. On Windows, set RUST_LOG=trace
2. Launch deepseek-tui --verbose
3. Observe tracing log lines from stderr overwriting the alt-screen buffer

Changes

• crates/tui/src/logging.rs: Remove RUST_LOG check from verbose output gate
• Add documentation comment explaining why RUST_LOG is intentionally excluded

Closes #1774