src/vs/editor/common/model/textModel.ts/TextModel/modifyPosition
class TextModel:	public modifyPosition(rawPosition: IPosition, offset: number): Position {
		this._assertNotDisposed();
		const candidate = this.getOffsetAt(rawPosition) + offset;
		return this.getPositionAt(Math.min(this._buffer.getLength(), Math.max(0, candidate)));
	}