Question

I am having trouble with incorrectly painted corners when drawing VCL-styled window elements. On styles that have rounded corners, I get a white background in the space between the control’s bounding rect and the style’s rounded window corner.

The above image was run using Aqua Light Slate, but any style with rounded corners will show this same problem. What am I missing?

type
  TSample = class(TCustomControl)
  protected
    procedure Paint; override;
  end;

{ TForm1 }
procedure TForm1.FormCreate(Sender: TObject);
var
  R: TRect;
  S: TSample;
begin
  R := ClientRect;
  InflateRect(R, -20, -20);
  S := TSample.Create(Application);
  S.Parent := Self;
  S.BoundsRect := R;
end;

{ TSample }
procedure TSample.Paint;
var
  Details: TThemedElementDetails;
begin
  Details := StyleServices.GetElementDetails(twCaptionActive);
  StyleServices.DrawParentBackground(Self.Handle, Canvas.Handle, Details, False);
  StyleServices.DrawElement(Canvas.Handle, Details, ClientRect);
end;

Answer 1

Ok, I spend some minutes in you question and I found the answer. The key to draw the rounded corners, is call the StyleServices.GetElementRegion function to get the region and then use the SetWindowRgn function to apply the region to the control.

check this sample

procedure TSample.Paint;
var
  Details : TThemedElementDetails;
  Region  : HRgn;
  LRect   : TRect;
begin
  Details := StyleServices.GetElementDetails(twCaptionActive);
  LRect := Rect(0, 0, Width, Height);
  StyleServices.GetElementRegion(Details, LRect, Region);
  SetWindowRgn(Handle, Region, True);
  StyleServices.DrawParentBackground(Self.Handle, Canvas.Handle, Details, False);
  StyleServices.DrawElement(Canvas.Handle, Details, ClientRect);
end;

And this is the result