Trackbar Notification Messages

A trackbar notifies its parent window of user actions by sending the parent WM_HSCROLL or WM_VSCROLL messages. 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, and the high-order word specifies the position of the slider. The lParam parameter is the handle of 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

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