Dragging a TPaintBox while keeping its content static

Question

We are having a VCL TPaintBox on which we display video. Its parent is a TScrollbox.

When zooming, the PaintBox gets larger than the scrollbox, so parts of the image is hidden.

We have implemented the ability to drag the paintbox, using the mouse, within the scrollbox, which works fine.

However, whenever the paintBox is moved, i.e. its left or top property is changed, its content gets erased.

Question is, is there a way to prevent the paintbox to erase its content while moving it?

I created a simple VCL Forms example with a paintbox inside a scroll box, code below. When clicking on the keyboard, a line is painted on the paintbox.

The example shows, that without constant painting, the content of the paintbox gets erased upon moving itself. Question is, how to avoid, or deal with?

#ifndef Unit1H
#define Unit1H
#include <System.Classes.hpp>
#include <Vcl.Controls.hpp>
#include <Vcl.StdCtrls.hpp>
#include <Vcl.Forms.hpp>
#include <Vcl.ExtCtrls.hpp>
#include "RzButton.hpp"

class TForm1 : public TForm
{
    __published:
        TPaintBox *PB;
        TScrollBox *ScrollBox1;
        void __fastcall ObjMouseDown(TObject *Sender, TMouseButton     Button, TShiftState Shift, int X, int Y);
        void __fastcall ObjMouseMove(TObject *Sender, TShiftState Shift, int X, int Y);
        void __fastcall ObjMouseUp(TObject *Sender, TMouseButton Button,  TShiftState Shift, int X, int Y);
        void __fastcall PBPaint(TObject *Sender);
        void __fastcall FormKeyDown(TObject *Sender, WORD &Key, TShiftState Shift);

    public:
        __fastcall TForm1(TComponent* Owner);

        bool mIsDragging;
        TPoint mMouseDownLocation;
        TPoint mDragStartingLocation;
};
extern PACKAGE TForm1 *Form1;
#endif

and source

#include <vcl.h>
#pragma hdrstop
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma link "RzButton"
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
: TForm(Owner)
{}
//---------------------------------------------------------------------------
void __fastcall TForm1::ObjMouseDown(TObject *Sender, TMouseButton Button, TShiftState Shift,
      int X, int Y)
{
    mIsDragging = true;
    mMouseDownLocation = Mouse->CursorPos;
    mDragStartingLocation = TPoint(PB->Left, PB->Top);
}

//---------------------------------------------------------------------------
void __fastcall TForm1::ObjMouseMove(TObject *Sender, TShiftState Shift,
      int X, int Y)
{
     if(mIsDragging)
     {
        PB->Left = mDragStartingLocation.X + (Mouse->CursorPos.X - mMouseDownLocation.X);
        PB->Top = mDragStartingLocation.Y  + (Mouse->CursorPos.Y - mMouseDownLocation.Y);
     }
}

//---------------------------------------------------------------------------
void __fastcall TForm1::ObjMouseUp(TObject *Sender, TMouseButton Button,
      TShiftState Shift, int X, int Y)
{
     mIsDragging = false;
}

//---------------------------------------------------------------------------
void __fastcall TForm1::PBPaint(TObject *Sender)
{
//    PB->Canvas->Pen->Color = clRed;
//    PB->Canvas->Pen->Width = 3;
//    PB->Canvas->MoveTo(50,50);
//    PB->Canvas->LineTo(150, 150);
}

//---------------------------------------------------------------------------
void __fastcall TForm1::FormKeyDown(TObject *Sender, WORD &Key, TShiftState Shift)
{
  PB->Canvas->Pen->Color = clRed;
  PB->Canvas->Pen->Width = 3;
  PB->Canvas->MoveTo(50,50);
  PB->Canvas->LineTo(150, 150);
}