src/vs/editor/browser/controller/textAreaHandler.ts/TextAreaHandler/_ensureReadOnlyAttribute
class TextAreaHandler:	private _ensureReadOnlyAttribute(): void {
		const options = this._context.configuration.options;
		// When someone requests to disable IME, we set the "readonly" attribute on the <textarea>.
		// This will prevent composition.
		const useReadOnly = !IME.enabled || (options.get(EditorOption.domReadOnly) && options.get(EditorOption.readOnly));
		if (useReadOnly) {
			this.textArea.setAttribute('readonly', 'true');
		} else {
			this.textArea.removeAttribute('readonly');
		}
	}