src/vs/editor/browser/viewParts/lines/viewLines.ts/ViewLines/getLineWidth
class ViewLines:	public getLineWidth(lineNumber: number): number {
		const rendStartLineNumber = this._visibleLines.getStartLineNumber();
		const rendEndLineNumber = this._visibleLines.getEndLineNumber();
		if (lineNumber < rendStartLineNumber || lineNumber > rendEndLineNumber) {
			// Couldn't find line
			return -1;
		}

		return this._visibleLines.getVisibleLine(lineNumber).getWidth();
	}