TrackMeasureOverride 方法

The desired size of a Track is the width (if vertically oriented) or height (if horizontally oriented) of the Thumb. When ViewportSize is NaN: The thumb is measured to find the other dimension. Otherwise: Zero size is returned; Track can scale to any size along its children. This means that it will occupy no space (and not display) unless made larger by a parent or specified size.

protected override Size MeasureOverride(
	 in Size availableSize
)