How to repaint canvas just in time?

Question

The problem is: I draw some rectangle on the desktop, while mouse moving(rectangle size increases) I dont have lags, artefacts, etc, All is good:

But when I resize rectangle to lower than it was beed, I have the artefact:

The red rectangle is the real rectangle, all other is the bug.

The perfect solution is redraw the canvas, but I can't do it all time while mouse is moving.

Is there solution to do some when mouse absolute stops after moving?

Update

The code:

    unit Unit2;

interface

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

type
  TForm2 = class(TForm)
    Timer1: TTimer;
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure FormCreate(Sender: TObject);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
    isDown: Boolean;
    downX, downY: Integer;
  public
    { Public declarations }
    Bild: TBitMap;
  end;

implementation

{ Form props: BorderStyle= bsNone AlphaBlend true, 150 Transparentcolor = true, clBlack }

{$R *.dfm}

procedure TForm2.FormCreate(Sender: TObject);
begin
  Bild := TBitMap.Create;
end;

procedure TForm2.FormDestroy(Sender: TObject);
begin
  Bild.Free;
end;

procedure TForm2.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  isDown := true;
  downX := X;
  downY := Y;
end;

procedure TForm2.FormMouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
const
  cVal = 4;
begin
  if isDown then
  begin
    Self.Canvas.Lock;
    Self.Repaint;
    Self.Canvas.Pen.Color := clNone;
    Self.Canvas.Pen.Width := 1;

    Self.Canvas.Pen.Style := psDot;
    //Self.Canvas.Pen.Mode := pmNotCopy;
    Self.Canvas.Brush.Color := clGreen;
    Self.Canvas.Rectangle(downX, downY, X, Y);
    Self.Canvas.Pen.Style := psSolid;
    Self.Canvas.Brush.Color := clNone;
    Self.Canvas.Unlock;
    { Self.Canvas.Rectangle(downX - cVal, downY - cVal, downX + cVal, downY + cVal);
     Self.Canvas.Rectangle(X - cVal, Y - cVal, X + cVal, Y + cVal);
     Self.Canvas.Rectangle(X - cVal, downY - cVal, X + cVal, downY + cVal);
     Self.Canvas.Rectangle(downX - cVal, Y - cVal, downX + cVal, Y + cVal);

     Self.Canvas.Rectangle(downX - cVal, (downY + Y) div 2 - cVal, downX + cVal,
       (downY + Y) div 2 + cVal);
     Self.Canvas.Rectangle(X - cVal, (downY + Y) div 2 - cVal, X + cVal,
       (downY + Y) div 2 + cVal);

     Self.Canvas.Rectangle((downX + X) div 2 - cVal, downY - cVal,
       (downX + X) div 2 + cVal, downY + cVal);
     Self.Canvas.Rectangle((downX + X) div 2 - cVal, Y - cVal, (downX + X) div 2 + cVal,
       Y + cVal);   }
  end;
end;

function CaptureRect(aRect: TRect; out aBmp: TBitmap): Boolean;
var
  ScreenDC: HDC;
begin
  Result := False;
  try
    with aBmp, aRect do
    begin
      Width := Right - Left;
      Height := Bottom - Top;
      ScreenDC := GetDC(0);
      try
        BitBlt(Canvas.Handle, 0, 0, Width, Height, ScreenDC, Left, Top, SRCCOPY);
      finally
        ReleaseDC(0, ScreenDC);
      end;
    end;
  except
  end;
end;

procedure TForm2.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
var
  r: TRect;
begin
  isDown := false;
  r.Left := downX;
  r.Top := downY;
  r.Right := X;
  r.Bottom := Y;
  CaptureRect(r, Bild);
  Self.Close;
end;

end.

Answer 1

Your problem is that you are painting in the wrong place. Stop painting in the OnMouseMove event handler. Move the painting code to a paint handler. For example the form's OnPaint handler.

Then, in the OnMouseMove event handler, and indeed OnMouseDown and OnMouseUp, call Invalidate on the form, or the Win32 InvalidateRect function, to force a paint cycle.