Editor: event listeners never cleaned up on unmount, deprecated componentWillUpdate

[avinxshKD]

p5.js version

No response

What is your operating system?

None

Web browser and version

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Actual Behavior

client/modules/IDE/components/Editor/index.jsx has been leaking event
listeners since forever. Line 1 even has a TODO to convert it to a
functional component, so this has been known but nobody fixed the leaks.

Problems:

1. componentDidMount registers click, keydown, and change (debounced)
   listeners on the CodeMirror instance. componentWillUnmount only removes
   keyup. The rest are gone forever. click and keydown are anonymous arrow
   functions so they can't even be removed with .off() as-is.

componentWillUnmount() {
  if (this._cm) {
    this._cm.off('keyup', this.handleKeyUp); // only this gets cleaned up
  }
  this.props.provideController(null);
}

2. componentWillUpdate (line ~260) is deprecated since React 16.3.
   componentDidUpdate already exists, the logic should just move there.

3. The debounced change handler is never cancelled on unmount, so it can
   fire after the component is gone.

Fix:

• Convert anonymous listeners to named methods so they can be properly
  unregistered
• Cleanup all listeners in componentWillUnmount
• Move componentWillUpdate logic into componentDidUpdate
• Cancel debounced handler on unmount

[avinxshKD]

@raclim pls assign