Track::Thumb Property
Assembly: PresentationFramework (in PresentationFramework.dll)
The placement of the Thumb control corresponds to the Value of the Track control. The size of the Thumb control in a Track control reflects the amount of content that is currently visible. The size of the Thumb control of the Track is computed in the following ways:
If the ViewportSize property is not specified, the size of the Thumb is a fixed default value. This is how the Thumb of a Track functions inside a Slider control.
If the ViewportSize property is specified and the size of the content is greater than the ViewportSize, the following formula is used:
ThumbSize = TrackLength * ViewportSize / (Maximum – Minimum + ViewportSize)
Where the parameters are defined as follows:
ThumbSize
The size of the Thumb control in the same units as the TrackLength.
TrackLength
The length of the Track.
ViewportSize
The size of the scrollable area that is visible in the content units, such as the number of pages of a document.
Maximum
The maximum value of the content in content units, such as page 10 of a 10-page document.
Minimum
The minimum value of the content in content units, such as page 1 of a document.
The following illustration shows a visual representation of some of these parameters in a ScrollBar control.

If the size of the content is less than a ViewportSize, the Thumb does not appear and the Visibility property of Thumb is set to false.
The minimum sizes for a Thumb control are determined by two system parameters, VerticalScrollBarButtonHeight and HorizontalScrollBarThumbWidth. The minimum size for a Thumb control in a vertical Track is 1/2 * VerticalScrollBarButtonHeight and the minimum size for a Thumb control in a horizontal Track is 1/2 * HorizontalScrollBarThumbWidth.
The following example shows how the Thumb style is defined inside the ScrollBarControlTemplate when the Orientation property is Vertical.
Available since 3.0