How to pull a MDI child window out of the main form?

Question

I want to create a MDI application with it's own task bar so the user can have fast access to the child windows he/she wants to bring to front. Then I had the idea that an user who works with two or more monitors could drag on of the child windows from inside the main form of my application into outside of it, into another monitor for example.

How can it be done?

Answer 1

Maybe this example MDI client form code serves inspiration:

unit Unit3;

interface

uses
  Windows, Messages, Controls, Forms;

type
  TForm3 = class(TForm)
  private
    FSizing: Boolean;
    procedure WMNCMouseLeave(var Message: TMessage);
      message WM_NCMOUSELEAVE;
    procedure WMWindowPosChanged(var Message: TWMWindowPosChanged);
      message WM_WINDOWPOSCHANGED;
  protected
    procedure CreateParams(var Params: TCreateParams); override;
    procedure Resize; override;
  end;

implementation

{$R *.dfm}

{ TForm3 }

var
  FDragging: Boolean = False;

procedure TForm3.CreateParams(var Params: TCreateParams);
begin
  inherited CreateParams(Params);
  if FormStyle = fsNormal then
    Params.ExStyle := Params.ExStyle or WS_EX_APPWINDOW
  else
    Params.ExStyle := Params.ExStyle and not WS_EX_APPWINDOW;
end;

procedure TForm3.Resize;
begin
  inherited Resize;
  FSizing := True;
end;

procedure TForm3.WMNCMouseLeave(var Message: TMessage);
begin
  inherited;
  FDragging := False;
end;

procedure TForm3.WMWindowPosChanged(var Message: TWMWindowPosChanged);
var
  P: TPoint;
  F: TCustomForm;
  R: TRect;
begin
  inherited;
  if not FDragging and not FSizing and not (fsShowing in FormState) and
    (WindowState = wsNormal) then
  begin
    F := Application.MainForm;
    P := F.ScreenToClient(Mouse.CursorPos);
    R := F.ClientRect;
    InflateRect(R, -5, -5);
    if not PtInRect(R, P) and (FormStyle = fsMDIChild) then
    begin
      FDragging := True;
      FormStyle := fsNormal;
      Top := Top + F.Top;
      Left := Left + F.Left;
    end
    else if PtInRect(R, P) and (FormStyle = fsNormal) then
    begin
      FDragging := True;
      FormStyle := fsMDIChild;
    end;
  end;
  FSizing := False;
end;

end.