src/vs/editor/browser/controller/mouseTarget.ts/CharWidthReader/getCharWidth
class CharWidthReader:	public getCharWidth(char: string, font: string): number {
		const cacheKey = char + font;
		if (this._cache[cacheKey]) {
			return this._cache[cacheKey];
		}

		const context = this._canvas.getContext('2d')!;
		context.font = font;
		const metrics = context.measureText(char);
		const width = metrics.width;
		this._cache[cacheKey] = width;
		return width;
	}