PaintBox vs. Canvas dimensions

[lucamar]

Antecedents
As shown in the attached image, the Width and Height of a TPaintBox and its Canvas are different, which means this code will not do the expected:

Code: Pascal  [Select][+][-]

1.   with PaintBox.Canvas do begin
2.     Brush.Color := clCream;
3.     FillRect(0, 0, Width, Height);
4.     Frame(0, 0, Width, Height);
5.   end;

while this one wiill do it:

Code: Pascal  [Select][+][-]

1.   with PaintBox do begin
2.     Canvas.Brush.Color := clCream;
3.     Canvas.FillRect(0, 0, Width, Height);
4.     Canvas.Frame(0, 0, Width, Height);
5.   end;

In case it matters, the PaintBox is aligned to Client with "BorderSpacing.Around = 6". The dimensions of the canvas are the same than those of the form's client area.

Question:
Is this a known (and "won't solve") problem or should I report in the bug tracker? Or is it how it's supposed to work?

Environment
Linux-i686 3.2.0/GTK 2.24.10
Lazarus/FPC: 2.0.2/3.0.4 (vanilla gtk install)

[wp]

As shown in the attached image, the Width and Height of a TPaintBox and its Canvas are different, which means this code will not do the expected:

Code: Pascal  [Select][+][-]

1.   with PaintBox.Canvas do begin
2.     Brush.Color := clCream;
3.     FillRect(0, 0, Width, Height);
4.     Frame(0, 0, Width, Height);
5.   end;

I always thought that the Canvas is infinite, it does not have neither a Width nor a Height. In this case, Width and Height in above code would refer to the parameters of the Form ( a dangerous usage of the "with" syntax BTW).

But I am wrong, there are properties TCanvas.Width and TCanvas.Height indeed. Peeking into the source code shows

Code: Pascal  [Select][+][-]

1. function TCanvas.GetWidth: integer;
2. var
3.   p: TPoint;
4. begin
5.   if HandleAllocated then begin
6.     GetDeviceSize(Handle,p);
7.     Result:=p.x;
8.   end else
9.     Result:=0;
10. end;

and GetDeviceSize leads into a widgetset-related method. In case of Windows, this calls "WindowFromDC" which in fact seems to do what I suspected above: it links the extent of the canvas to the exent of the form. Of course, every widgetset may handle this case differently. So, for device-independent code, I would refrain from referring to Canvas.Width and Canvas.Height, like I always did.

Just my 2 cents...

[lucamar]

Yes, as I said the PaintBox' Canvas is getting the dimensions either of the form client area or of the paintbox before applying BorderSpacing.

I suspect the former, rather than the later, because TPaintBox is a TGraphicControl.

The question is: Is this already known and as should be or a bug?

BTW: Yes, it's a rather dangerous use of "with" but it's just a small test and I was in something of a hurry
And I checked it did what it was suposed to do ...
(famous last words, huh? )