src/vs/editor/browser/controller/textAreaHandler.ts/TextAreaHandler/_getCharacterBeforePosition
class TextAreaHandler:	private _getCharacterBeforePosition(position: Position): string {
		if (position.column > 1) {
			const lineContent = this._context.viewModel.getLineContent(position.lineNumber);
			const charBefore = lineContent.charAt(position.column - 2);
			if (!strings.isHighSurrogate(charBefore.charCodeAt(0))) {
				return charBefore;
			}
		}
		return '';
	}