TImage image is being truncated

I have written a Print Preview option for a larger application. Mostly it works to my satisfaction, except sometimes the image is truncated.

The code below is a much cut-down version that shows the problem. I have a TPanel on which I am putting a TImage, which for now is just a white rectangle with a red line going from the top left corner to the bottom right.

In the larger application, I want to choose between A4 portrait size and A4 landscape. My little app here has two buttons to choose which page size is required.

Here's the problem. When I start the app and select Portrait, it works correctly:

But, when I then click Landscape, the image is truncated. You can see which way the red line is going:

On the other hand, when I stop the app and restart it and select Landscape, it works ok:

When I then click Portrait, it is this image that is now truncated:

How can I get this to work?

My code is below:

const float A4width = 8.27/4, A4height = 11.69/4;

__fastcall TForm1::TForm1(TComponent* Owner) : TForm(Owner)
{
    image = new TImage(Owner);
    image->Parent = Panelpreview;
    image->Left = 16;
    image->Top = 16;
}

// Portrait button click
void __fastcall TForm1::BportraitClick(TObject *Sender)
{
    fheight = A4height;
    fwidth = A4width;

    showPreview();
}

//Landscape button click
void __fastcall TForm1::BlandscapeClick(TObject *Sender)
{
    fheight = A4width;
    fwidth = A4height;

    showPreview();
}

// Draw the picture
void TForm1::showPreview()
{
    int pagewidth, pageheight;
    TRect imagerect;

    pagewidth = fwidth * Canvas->Font->PixelsPerInch;
    pageheight = fheight * Canvas->Font->PixelsPerInch;
    imagerect = TRect(0, 0, pagewidth, pageheight);

    image->Width = pagewidth;
    image->Height = pageheight;

    image->Canvas->Brush->Color = clWhite;
    image->Canvas->FillRect(imagerect);
    image->Canvas->Pen->Color = clRed;
    image->Canvas->MoveTo(0, 0);
    image->Canvas->LineTo(pagewidth, pageheight);
}