Adding a Ctrl+arrow accelerator for moving the trackbar by just one unit, part 2: Second try

Last time, we looked at how we could add support to the trackbar
so that Ctrl+arrow moved the thumb by one unit,
even if the line size was set to a larger value.
We tried doing this by subclassing the control and adding
additional keyboard handling,
but this turned into a bit of a mess because of all the special
cases in the trackbar to accommodate various usage patterns.

What we really want to do is let the trackbar do all its
keyboard processing, and step in just before it moves the thumb,
so we can move it by a different amount if the Ctrl
key is held down.

Fortunately, there's a notification for this.

Unfortunately, it requires version 6 of the common controls.

Fortunately, version 6 of the common controls is included in all
versions of Windows still in support.

Take our program from last time, but stop before we added the
Trackbar­Key­Proc.
(Delete the
Trackbar­Key­Proc and the calls to
Set­Window­Subclass and
Remove­Window­Subclass.)

Instead, add this code:

#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.

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

{

 if (pnm->hwndFrom == g_hwndChild &&

     pnm->code == TRBN_THUMBPOSCHANGING &&

     GetKeyState(VK_CONTROL) < 0) {

  auto ptpc = (NMTRBTHUMBPOSCHANGING*)pnm;

  switch (ptpc->nReason) {

  case TB_LINEUP:

  case TB_LINEDOWN:

    int pos = (int)SendMessage(pnm->hwndFrom, TBM_GETPOS, 0, 0);

    pos += (ptpc->nReason == TB_LINEUP) ? -1 : +1;

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

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

  }

 }

 return 0;

}

    HANDLE_MSG(hwnd, WM_NOTIFY, OnNotify);

The
TRBN_THUMB­POS­CHANGING
notification is sent before the trackbar moves the thumb.
and
the nReason tells you why the trackbar wants to
move the thumb.¹
If the Ctrl key is held down, and the reason is
either a line-up or a line-down, then we fetch the current
trackbar position, adjust it by one unit, and set that as the
new trackbar position.
We then return TRUE to tell the trackbar that it shouldn't
move the trackbar thumb (because we moved it).

(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.)

Responding to the notification leaves the trackbar to deal with
recognizing the keyboard keys and taking the various
trackbar configuration settings into account in order to convert
them to scroll actions.
We then detect the change position by one line
action and apply our special thumb motion if the Ctrl
key is held down,
leaving the trackbar to manage the keyboard cues and other
accessibility states.

¹
There's also a dwPos that tells you where
the thumb is moving to,
but we are more interested in where the thumb is moving from.