Flex :: Limit View Area (viewport?) Of S:TileGroup

I have a s:TileGroup as follows:

<s:TileGroup id="myTileGroup"
clipAndEnableScrolling="true" focusEnabled="true"
horizontalGap="4"

[Code]....

As items are added to the TileGroup it "overflows" its' bounds, items are displayed below, and when scrolled above, the 3x3 grid.

Is there a way to "mask" or otherwise hide these items?

I looking at the properties of TileGroup, I've tried to emplement the "verticalScrollPosition" and "bottom" properties - these don't do it.

Setting the "height" property effectively creates a bottom limit for the group, but when scrolled items are still displayed above the "padding" area.