New MDI Child Forms By Zarko Gajic, About.com

If you are creating MDI applications using Delphi, you must have noticed some "quirks" or issues that you cannot simply handle / fix from your code.
MDI interface was designed in the days of Windows 3 (some 10+ years ago) and it was designed with a single type of application in mind: a parent window that hosts multiple instances of the same class of "document" window (just think of you first MS Word).

 

Nasty MDI Child Resizing Animation

When creating (to show) an MDI child an animation of resizing will take place. This animation might look ugly if the code executed during the creation of your MDI child takes some time to process. Even if WindowState property was set to wsMaximized when an MDI child is created it will be created using Default Pos at X, Y coordinates where you left your form at design time.
Windows simply insists on creating MDI children visible and at a default position.
After creation, the MDI child will get maximized but an ugly animation will take place.

 

Quickly Create MDI Children - Eliminate "Create & Resize" Animation

To eliminate the MDI child creation and resizing animation you can send a special message to the MDI parent form, WM_SETREDRAW. The WM_SETREDRAW can be sent to a window to enable changes in that window to be redrawn or to prevent changes in that window from being redrawn. To prevent the animation flicker, have the next code in your MDI Parent form's unit:

TMDIMainForm = class(TForm)
private
  fLockClientWindowUpdateCount: Integer;
public
  constructor Create(aOwner: TComponent) ; override;
  procedure LockClientWindowUpdate;
  procedure UnlockClientWindowUpdate;
end;

...

constructor TMDIMainForm.Create(aOwner: TComponent) ;
begin
  inherited Create(aOwner) ;
  fLockClientWindowUpdateCount := 0;
end;

procedure TMDIMainForm.LockClientWindowUpdate;
begin
  if fLockClientWindowUpdateCount = 0 then SendMessage(ClientHandle, WM_SETREDRAW, 0, 0) ;
  Inc(fLockClientWindowUpdateCount) ;
end;

procedure TMDIMainForm.UnlockClientWindowUpdate;
begin
  Dec(fLockClientWindowUpdateCount) ;
  if fLockClientWindowUpdateCount = 0 then
  begin
    SendMessage(ClientHandle, WM_SETREDRAW, 1, 0) ;
    RedrawWindow(ClientHandle, nil, 0, RDW_FRAME or RDW_INVALIDATE or RDW_ALLCHILDREN or RDW_NOINTERNALPAINT)
  end
end;

Now, when you need to create (and show) an MDI client form(s), just call LockClientWindowUpdate and UnlockClientWindowUpdate. If a client window takes some time to create, you can change the cursor to let the user something (form creation) is going on:

LockClientWindowUpdate;
Screen.Cursor := crHourGlass;
try
  Application.CreateForm(MDIChildForm) ;
finally
  Screen.Cursor := crDefault;
  UnlockClientWindowUpdate;
end;

That's it. Now your MDI child forms will load faster and without the confusing animation.