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procedure TForm1.CheckBox1Click(Sender: TObject);
begin
if CheckBox1.Checked then
SetWindowPos(Handle, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOSIZE or SWP_NOMOVE)
else
SetWindowPos(Handle, HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOSIZE or SWP_NOMOVE);
end;//ʹÓÃSelf.FormStyle := fsStayOnTop;»áʹ½çÃæÉÁ˸һÏÂ

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The SetWindowPos function changes the size, position, and Z order of a child, pop-up, or top-level window. Child, pop-up, and top-level windows are ordered according to their appearance on the screen.
BOOL SetWindowPos(
    HWND hWnd, // handle of window
    HWND hWndInsertAfter, // placement-order handle
    int X, // horizontal position
    int Y, // vertical position
    int cx, // width
    int cy, // height
    UINT uFlags  // window-positioning flags
   );
HWND_NOTOPMOST Places the window above all non-topmost windows (that is, behind all topmost windows). This flag has no effect if the window is already a non-topmost window.
HWND_TOPMOST Places the window above all non-topmost windows. The window maintains its topmost position even when it is deactivated.
SWP_NOMOVE Retains the current position (ignores the X and Y parameters).
SWP_NOSIZE Retains the current size (ignores the cx and cy parameters).