Notifications
NM_CUSTOMDRAW (trackbar)
Sent by a trackbar control to notify its parent windows about drawing operations. This notification is sent in the form of a WM_NOTIFY message.

NM_RELEASEDCAPTURE (trackbar)
Notifies a trackbar control's parent window that the control is releasing mouse capture. This notification is sent in the form of a WM_NOTIFY message.

Trackbar Notification Messages
A trackbar notifies its parent window of user actions by sending the parent a WM_HSCROLL or WM_VSCROLL message. A trackbar with the TBS_HORZ style sends WM_HSCROLL messages. A trackbar with the TBS_VERT style sends WM_VSCROLL messages. The low-order word of the wParam parameter of WM_HSCROLL or WM_VSCROLL contains the notification code. For the TB_THUMBPOSITION and TB_THUMBTRACK notifications, the high-order word of the wParam parameter specifies the position of the slider. For all other notifications, the high-order word is zero; send the TBM_GETPOS message to determine the slider position. The lParam parameter is the handle to the trackbar.

The system sends the TB_BOTTOM, TB_LINEDOWN, TB_LINEUP, and TB_TOP notification messages only when the user interacts with a trackbar by using the keyboard. The TB_THUMBPOSITION and TB_THUMBTRACK notification messages are only sent when the user is using the mouse. The TB_ENDTRACK, TB_PAGEDOWN, and TB_PAGEUP notification messages are sent in both cases. The following table lists the trackbar notification messages and the events (virtual key codes or mouse events) that cause the notifications to be sent.

Notification message Reason sent
TB_BOTTOM        VK_END
TB_ENDTRACK      WM_KEYUP(the user released a key that sent a relevant virtual key code)
TB_LINEDOWN      VK_RIGHT or VK_DOWN
TB_LINEUP        VK_LEFT or VK_UP
TB_PAGEDOWN      VK_NEXT (the user clicked the channel below or to the right of the slider)
TB_PAGEUP        VK_PRIOR (the user clicked the channel above or to the left of the slider)
TB_THUMBPOSITION WM_LBUTTONUP following a TB_THUMBTRACK notification message
TB_THUMBTRACK    Slider movement (the user dragged the slider)
TB_TOP           VK_HOME

procedure TForm1.TrackBar1Scroll(Sender: PTrackbar; Code: Integer);
begin
 Form.Caption := Int2Str(Sender.Position);
end;