How to enable the mouse wheel in ScrollViewer

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There are areas in Silverlight's ScrollViewer where the mouse wheel doesn't work. The place where the control with the background color is specified seems to be effective, and the rest seems not to work.

ScrollViewer to enable the whole seems to be good to do as follows

<ScrollViewer Background="Transparent">

The color seems to be all right even if it is opaque or translucent.