Author: Jonas Bilinkevicius

I have a MDI application which maximizes the client forms as they are created. On 
Win9x I can stop the initial flash of the MDI child form being created by using 
LockWindowUpdate(Handle). However on Win XP this code doesn't work as predicted. 
The form updating is turned off, but the client form still draws its outline 
briefly in the MDI client space. So, does anyone know how I can get round this on 
Win XP?

Answer:

1   { ...}
2   TForm1 = class(TForm)
3   private
4     fLockClientUpdateCount: Integer;
5   public
6     constructor Create(aOwner: TComponent); override;
7     procedure LockClientUpdate;
8     procedure UnlockClientUpdate;
9   end;
10  
11  { ... }
12  
13  constructor TForm1.Create(aOwner: TComponent);
14  begin
15    inherited Create(aOwner);
16    fLockClientUpdateCount := 0;
17  end;
18  
19  procedure TForm1.LockClientUpdate;
20  begin
21    if fLockClientUpdateCount = 0 then
22      SendMessage(ClientHandle, WM_SETREDRAW, 0, 0);
23    Inc(fLockClientUpdateCount);
24  end;
25  
26  procedure TForm1.UnlockClientUpdate;
27  begin
28    Dec(fLockClientUpdateCount);
29    if fLockClientUpdateCount = 0 then
30    begin
31      SendMessage(ClientHandle, WM_SETREDRAW, 1, 0);
32      RedrawWindow(ClientHandle, nil, 0, RDW_FRAME or RDW_INVALIDATE or
33        RDW_ALLCHILDREN or RDW_NOINTERNALPAINT)
34    end;
35  end;


Now, simply call LockClientUpdate and UnlockClientUpdate instead of LockWindowUpdate.