When a BS_GROUPBOX style window is created, its background does not
The parent window of the BS_GROUPBOX style window has the
WS_CLIPCHILDREN style, which prevents the parent window from
erasing the group box's background.
Subclass the group box window to process the WM_ERASEBKGND message
by erasing its background. Listed below is a code fragment to
demonstrate this procedure.
This behavior is by design.
The WS_CLIPCHILDREN style causes a window to exclude the areas
occupied by child windows when the window paints its client area.
However, a BS_GROUPBOX style window is a static control that never
erases its background. Erasing the background removes any controls or
buttons that appear within the group box.
Therefore, when another child window is dropped over a group box and
subsequently dragged away, portions of that child window remain
visible in the group box's background. This problem does not occur
when the parent window does not have the WS_CLIPCHILDREN style.
The following code fragment should be placed in the group box's
subclass procedure. This code erases the background of the group box.
HBRUSH hBrush, hOldBrush;
HPEN hPen, hOldPen;
hDC = GetDC(hWnd);
// Obtain a handle to the parent window's background brush.
hBrush = GetClassWord(ghWnd, GCW_HBRBACKGROUND);
hOldBrush = SelectObject(hDC, hBrush);
// Create a background-colored pen to draw the rectangle
// borders, where gWindowColor is some globally defined
// COLORREF variable used to paint the window's background
hPen = CreatePen(PS_SOLID, 1, gWindowColor);
hOldPen = SelectObject(hDC, hPen);
// Erase the group box's background.
Rectangle(hDC, rect.left, rect.top, rect.right, rect.bottom);
// Restore the original objects before releasing the DC.
// Delete the created object.
// Instruct Windows to paint the group box text and frame.
InvalidateRect(hWnd, NULL, FALSE);
// Insert code here to instruct the contents of the group box
// to repaint as well.
return TRUE; // Background has been erased.