Author: Jonas Bilinkevicius

How can I show the system menu of a window at the position of the mouse cursor and 
not at the window's title bar?

Answer:

Solve 1:

The problem is that the system menu sends WM_SYSCOMMAND messages to the window 
identified by Handle, and you are probably looking for WM_COMMAND messages.

1   r := integer(TrackPopupMenuEx(GetSystemMenu(handle, false), TPM_LEFTALIGN 
2   or
3     TPM_RETURNCMD or TPM_RIGHTBUTTON or TPM_HORIZONTAL or
4     TPM_VERTICAL, x, y, handle, nil));
5   SendMessage(handle, WM_SYSCOMMAND, r, 0);



Solve 2:

Well, you can pop the system menu up where you want using code like the one below:

6   procedure TForm1.SpeedButton1Click(Sender: TObject);
7   var
8     h: HMENU;
9   begin
10    h := GetSystemMenu(handle, false);
11    TrackPopupMenu(h, TPM_LEFTALIGN or TPM_LEFTBUTTON, speedbutton1.Left +
12      clientorigin.X, speedbutton1.Top + speedbutton1.Height + clientorigin.y, 0,
13        handle, nil);
14  end;


The problem is that the menu will not work this way. If you use TrackPopupMenu to 
show the menu its items will send WM_COMMAND messages to the form when clicked by 
the user. But the form expects WM_SYSCOMMAND messages from the system menu. So you 
have to trap the WM_COMMAND messages, figure out which of them come from the menu 
(there will be lots of others, from buttons and the like) and translate them into 
WM_SYSCOMMAND.

15  { ... }
16  private
17  { Private declarations }
18  
19  procedure WMCommand(var msg: TWMCommand); message WM_COMMAND;
20  { ... }
21  
22  procedure TForm1.WMCommand(var msg: TWMCommand);
23  begin
24    if msg.NotifyCode = 0 then {message comes from a menu}
25      if msg.ItemID >= SC_SIZE then
26      begin {looks like system menu item}
27        PostMessage(handle, WM_SYSCOMMAND, TMessage(msg).WParam,
28          TMessage(msg).LParam);
29        Exit;
30      end;
31    inherited;
32  end;



Solve 3:

There is an undocumented Windows message (Message ID:$313) that can do it:
33  
34  procedure TForm1.Button1Click(Sender: TObject);
35  const
36    WM_POPUPSYSTEMMENU = $313;
37  begin
38    SendMessage(Handle, WM_POPUPSYSTEMMENU, 0,
39      MakeLong(Mouse.CursorPos.X, Mouse.CursorPos.Y));
40  end;