Code highlighting failed, falling back to plain text warning logged on exit (destroy race)

[HARIOM-JHA01]

Summary

When an app using @opentui/core exits, TreeSitterClient.destroy() rejects any in-flight highlightOnce() requests. CodeRenderable treats those rejections as genuine highlighting failures and logs a misleading warning:

Code highlighting failed, falling back to plain text: Error: TreeSitter client destroyed

This occurs during normal shutdown, so nothing is actually wrong. As a result, applications such as opencode print this warning on every exit (see anomalyco/opencode#36454).

Reproduction

1. Run any OpenTUI app that renders a CodeRenderable (or markdown containing code).
2. Exit the application while syntax highlighting is still in progress (e.g. Ctrl+C or /exit).
3. Observe the warning printed after teardown.

Root Cause

During shutdown, TreeSitterClient.destroy() rejects all pending highlight requests with:

new Error("TreeSitter client destroyed")

CodeRenderable.startHighlight() catches every rejected highlightOnce() call and assumes it represents a real highlighting failure, logging:

console.warn(
  "Code highlighting failed, falling back to plain text:",
  error,
);

However, in this case the rejection is an expected consequence of teardown, not an actual error.

Expected Behavior

Shutdown-related cancellations should be treated as expected and should not:

• emit a warning, or
• trigger the normal highlighting failure path.

Actual highlighting failures (for example, parser initialization or worker errors) should continue to log warnings so genuine issues remain visible.