Strange problem with simple scroll bar

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.

Code:

---

        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.

I have found a way to get the results I expect, but I'm puzzled as to whether this is the usual way a scroll bar needs to be handled. I've never had this issue before, and I've used scroll bars many times. Or maybe I'm just getting feebleminded in my old age :(.

Initialization values:

Code:

---

#define DESIREDRANGE  112
#define PAGESIZ  48

        vsi.nMin = 0;
        vsi.nMax = DESIREDRANGE + PAGESIZ - 1;
        vsi.nPage = PAGESIZ;
        vsi.nPos = 0;

---

Handling the messages:

Code:

---

        case WM_VSCROLL:
                if ((HWND)lParam == hWvscrl)
                {
                        switch (LOWORD(wParam))
                        {
                                case SB_LINEDOWN:
                                        vsi.nPos++;
                                        break;

                                case SB_LINEUP:
                                        vsi.nPos--;
                                        break;

                                case SB_PAGEDOWN:
                                        vsi.nPos += PAGESIZ;
                                        break;

                                case SB_PAGEUP:
                                        vsi.nPos -= PAGESIZ;
                                        break;

                                case SB_THUMBTRACK:
                                        vsi.nPos = HIWORD(wParam);
                                        break;
                        }
                        if (vsi.nPos > DESIREDRANGE) vsi.nPos = DESIREDRANGE;
                        if (vsi.nPos < vsi.nMin) vsi.nPos = vsi.nMin;
                        vsi.fMask = SIF_POS;
                        SetScrollInfo(hWvscrl, SB_CTL, &vsi, true);
                }

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