Jekyll `--incremental` flag and watch-mode staleness

Jekyll's --incremental flag is documented to accelerate repeated jekyll serve runs by skipping unchanged content during watch-mode rebuilds. However, the precise conditions under which stale content may persist without a full regeneration remain ambiguous, especially when multiple configuration files or plugin-injected assets are involved. This question addresses the boundary between incremental detection and guaranteed content freshness in a development workflow. The flag's interaction with file modification timestamps, directory watchers, and environment-specific _config.yml precedence is not explicitly defined in the official documentation, leaving developers to infer behavior through trial.

Does the --incremental flag respect file modification timestamps across all watched directories when --watch is active? How does the presence of a secondary _config.yml affect incremental rebuild decisions? What triggers a forced full rebuild despite the --incremental flag being set?