Smooth resizing in a borderless form/window in Delphi

Question

I am trying to resize a borderless form but when I increase the size using the right/bottom side, I get a gap between the border and the old client area that depends of the

Answer 1

I know this thread is fairly old, but it is one that people still struggle with.

The answer is simple, though. The problem is that trying to do resize stuff makes you want to use the form you are resizing as a reference. Don't do that.

Use another form.

Here is the complete source for a TForm that can help you. Make sure that this form has BorderStyle = bsNone. You probably also want to make sure it is not visible.

unit UResize;
{
  Copyright 2014 Michael Thomas Greer
  Distributed under the Boost Software License, Version 1.0
  (See accompanying file LICENSE.txt or copy
   at http://www.boost.org/LICENSE_1_0.txt )
}

//////////////////////////////////////////////////////////////////////////////
interface
//////////////////////////////////////////////////////////////////////////////

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

const
  ResizeMaskLeft   = $1;
  ResizeMaskTop    = $2;
  ResizeMaskWidth  = $4;
  ResizeMaskHeight = $8;

type
  TResizeForm = class( TForm )
    procedure FormMouseMove( Sender: TObject;      Shift: TShiftState; X, Y: Integer );
    procedure FormMouseUp(   Sender: TObject;
                             Button: TMouseButton; Shift: TShiftState; X, Y: Integer );
  private
    anchor_g: TRect;
    anchor_c: TPoint;
    form_ref: TForm;
    resize_m: cardinal;

  public
    procedure SetMouseDown( AForm: TForm; ResizeMask: cardinal );
  end;

var
  ResizeForm: TResizeForm;


//////////////////////////////////////////////////////////////////////////////
implementation
//////////////////////////////////////////////////////////////////////////////

{$R *.DFM}

//----------------------------------------------------------------------------
procedure TResizeForm.SetMouseDown( AForm: TForm; ResizeMask: cardinal );
  begin
  anchor_g.Left   := AForm.Left;
  anchor_g.Top    := AForm.Top;
  anchor_g.Right  := AForm.Width;
  anchor_g.Bottom := AForm.Height;
  anchor_c        := Mouse.CursorPos;
  form_ref        := AForm;
  resize_m        := ResizeMask;
  SetCapture( Handle )
  end;

//----------------------------------------------------------------------------
procedure TResizeForm.FormMouseMove(
  Sender: TObject;
  Shift:  TShiftState;
  X, Y:   Integer
  );
  var
    p: TPoint;
    r: TRect;
  begin
  if Assigned( form_ref ) and (ssLeft in Shift)
    then begin
         p := Mouse.CursorPos;
         Dec( p.x, anchor_c.x );
         Dec( p.y, anchor_c.y );

         r.Left   := form_ref.Left;
         r.Top    := form_ref.Top;
         r.Right  := form_ref.Width;
         r.Bottom := form_ref.Height;

         if (resize_m and ResizeMaskLeft)   <> 0 then begin r.Left   := anchor_g.Left   + p.x;  p.x := -p.x end;
         if (resize_m and ResizeMaskTop)    <> 0 then begin r.Top    := anchor_g.Top    + p.y;  p.y := -p.y end;
         if (resize_m and ResizeMaskWidth)  <> 0 then       r.Right  := anchor_g.Right  + p.x;
         if (resize_m and ResizeMaskHeight) <> 0 then       r.Bottom := anchor_g.Bottom + p.y;

         with r do form_ref.SetBounds( Left, Top, Right, Bottom )
         end
  end;

//----------------------------------------------------------------------------
procedure TResizeForm.FormMouseUp(
  Sender: TObject;
  Button: TMouseButton;
  Shift:  TShiftState;
  X, Y:   Integer
  );
  begin
  ReleaseCapture;
  form_ref := nil
  end;

end.

Now any borderless form in your application can be smoothly resized by hooking into ResizeForm with a simple

ResizeForm.SetMouseDown( self, (sender as TComponent).Tag );

A good place to put that is in the MouseDown event of whatever component(s) you are using to track the edges of your borderless form(s). (Notice how the Tag property is used to indicate what edge of your form you wish to drag/resize).

Oh, and set your form to DoubleBuffered = true to get rid of any remaining flicker.

This is just a small happiness I can give to you.