Strange problem with simple scroll bar

[cosmicvoid]

I have a dialog that contains a scroll bar (SB_CTL) that doesn't give the expected behavior. The first clue is that the size of the thumb button is wrong (too large) for the range/page proportions.

It is initialized to a range of 0 to 112, with a page size of 48.

	vsi.nMin = 0;
	vsi.nMax = 112;
	vsi.nPage = 48;
	vsi.nPos = 0;

When I grab the thumb, the position will go from 0 to 65, and no higher.
If click in the 'page down' area, it will go to 96, and no higher.
If I use the 'line up' and 'line down' buttons, it will go the full range 0 to 112, but the thumb position stops early. If the position is at 112 and I click on the thumb (at the bottom of the bar), the position immediately jumps to 65.

My experiments have yielded no joy, so I have distilled a minimum length test case, and I hope someone can please tell me why this code isn't working right. Using WinXP-SP3, VC++ 8.