Remove guard code to squelch repeated calls to connectedCallback()/disconnectedCallback().
Rename Widget lifecycle methods:
- preRender() --> beforeInitializeRendering()
- render() --> initializeRendering()
- postRender() --> afterInitializeRendering()
Change from JSHint to ESLint.
Leverage native (or polyfilled) Promise, Element#classList, and Element#matches.
Get rid of Selection#updateRenderers().