How do I prevent users from using the mouse to drag the trackbar thumb to positions that aren’t multiples of five? Part 2: Nudging the thumb position

Last time, we investigated one approach to the problem of
keeping a trackbar position on multiples of five:
Just divide everything by five!
(And then multiply by five when you read the value.)
This works great, except that accessibility tools report
the range as 0–20 in one-unit increments,
even though you are presenting it to the user as a range of
0–100 in five-unit increments.

So it's back to the drawing board.
This time, we'll leave the range at 0–100
and manipulate the trackbar position in the
TRBN_THUMB­POS­CHANGING
notification.

Take
our program from last time
and make the following changes:

#pragma comment(linker,

    ""/manifestdependency:type='win32'

    name='Microsoft.Windows.Common-Controls'

    version='6.0.0.0'

    processorArchitecture='*'

    publicKeyToken='6595b64144ccf1df'

    language='*'"")

This #pragma is

a quick way to enable version 6 of the common controls.

BOOL

OnCreate(HWND hwnd, LPCREATESTRUCT lpcs)

{

 g_hwndChild = CreateWindow(TRACKBAR_CLASS, TEXT(""),

    WS_CHILD | WS_VISIBLE | TBS_NOTIFYBEFOREMOVE,

   0, 0, 100, 100,

    hwnd, (HMENU)100, g_hinst, 0);
  SendMessage(g_hwndChild, TBM_SETLINESIZE, 0, 5);

  SendMessage(g_hwndChild, TBM_SETPAGESIZE, 0, 20);
  return TRUE;

}

The
TBS_NOTIFY­BEFORE­MOVE
style enables the
TRBN_THUMB­POS­CHANGING
notification, which we will take advantage of below.
We leave the range at its default of 0–100 and set
the line size and page size to the desired multiples of five.

LRESULT OnNotify(HWND hwnd, int idCtl, NMHDR* pnm)

{

 if (pnm->hwndFrom == g_hwndChild &&

     pnm->code == TRBN_THUMBPOSCHANGING) {

  auto ptpc = (NMTRBTHUMBPOSCHANGING*)pnm;

  auto pos = ptpc->dwPos;

  auto newpos = (pos + 2) / 5 * 5;

  if (pos != newpos) {

    SendMessage(pnm->hwndFrom, TBM_SETPOS, TRUE, newpos);

    return TRUE; // we moved the thumb, so the control doesn't have to

  }

 }

 return 0;

}

    HANDLE_MSG(hwnd, WM_NOTIFY, OnNotify);

When we are being notified that the thumb is about to move,
we take the proposed new position and round it to the nearest
multiple of five.
If that produces a value different from what the control would
have done, then we manually set the thumb position to the desired
multiple of five and return TRUE
to tell the trackbar that it shouldn't
move the trackbar thumb (because we moved it).

(And again, don't forget that if this is happening in a dialog box,
you need to use DWLP_MSG­RESULT
to make the dialog box return a nonzero value from its window
procedure.)