Ownerdraw TListBox child controls are not moved by scrolling - delphi

Question

procedure TForm1.ListBox1DrawItem(Control: TWinControl; Index: Integer;
Rect: TRect; State: TOwnerDrawState);
begin
inherited;
TListBox(Control).Canvas.FillRect(Rect);
TListBox(Control).Canvas.TextOut(Rect.Left+5, Rect.Top+8, TListBox(Control).Items[Index]);
if odSelected in State then
begin
Button.Left:=Rect.Right-80;
Button.Top:=Rect.Top+4;
Button.Visible:=true;
Button.Invalidate;
end;
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
ListBox1.DoubleBuffered:=true;
ListBox1.ItemHeight:=30;
ListBox1.Style:=lbOwnerDrawFixed;
Button:=TButton.Create(ListBox1);
Button.Parent:=ListBox1;
Button.DoubleBuffered:=true;
Button.Visible:=false;
Button.Width:=50;
Button.Height:=20;
Button.Caption:='BTN';
end;
The repaint problem only exists when using ScrollBar or sending WM_VSCROLL message to my ListBox. All normally drawn when I change selection by using keyboard arrows or mouse clicks. Problem also not exists when selected item are visible by scrolling and not leave visible area.
I think that Button.Top property still have an old value before DrawItem calls, and change (to -30px for example) later.

Answer 1

The problem is that you are using the OnDrawItem event to make changes to the UI (in this case, positioning the button). Do not do that, the event is for DRAWING ONLY.
I would suggest that you either:
subclass the ListBox to handle the WM_VSCROLL message and have your message handler reposition the button as needed.
var
PrevListBoxWndProc: TWndMethod;
procedure TForm1.FormCreate(Sender: TObject);
begin
PrevListBoxWndProc := ListBox1.WindowProc;
ListBox1.WindowProc := ListBoxWndProc;
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
ListBox1.WindowProc := PrevListBoxWndProc;
end;
procedure TForm1.PositionButton(Index: Integer);
var
R: TRect;
begin
if Index <= -1 then
Button.Visible := False
else
begin
R := ListBox1.ItemRect(Index);
Button.Left := R.Right - 80;
Button.Top := R.Top + 4;
Button.Visible := True;
end;
end;
var
LastIndex: Integer = -1;
procedure TForm1.ListBox1Click(Sender: TObject);
var
Index: Integer;
begin
Index := ListBox1.ItemIndex;
if Index <> LastIndex then
begin
LastIndex := Index;
PositionButton(Index);
end;
end;
procedure TForm1.ListBoxWndProc(var Message: TMessage);
begin
PrevListBoxWndProc(Message);
if Message.Msg = WM_VSCROLL then
PositionButton(ListBox1.ItemIndex);
end;
get rid of the TButton altogether. Use OnDrawItem to draw an image of a button (you can use DrawFrameControl() or DrawThemeBackground() for that) directly onto the ListBox, and then use the OnMouseDown/Up or OnClick event to check if the mouse is over the "button" and if so act accordingly as needed.
var
MouseX: Integer = -1;
MouseY: Integer = -1;
procedure TForm1.ListBox1DrawItem(Control: TWinControl; Index: Integer;
Rect: TRect; State: TOwnerDrawState);
var
R: TRect;
P: TPoint;
BtnState: UINT;
begin
TListBox(Control).Canvas.FillRect(Rect);
TListBox(Control).Canvas.TextOut(Rect.Left+5, Rect.Top+8, TListBox(Control).Items[Index]);
if not (odSelected in State) then Exit;
R := Rect(Rect.Right-80, Rect.Top+4, Rect.Right-30, Rect.Top+24);
P := Point(MouseX, MouseY);
BtnState := DFCS_BUTTONPUSH;
if PtInRect(R, P) then BtnState := BtnState or DFCS_PUSHED;
DrawFrameControl(TListBox(Control).Canvas.Handle, R, DFC_BUTTON, BtnState);
InflateRect(R, -4, -4);
DrawText(TListBox(Control).Canvas.Handle, 'BTN', 3, R, DT_CENTER or DT_VCENTER or DT_SINGLELINE);
end;
procedure TForm1.ListBox1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if Button <> mbLeft then Exit;
MouseX := X;
MouseY := Y;
ListBox1.Invalidate;
end;
procedure TForm1.ListBox1MouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if Button <> mbLeft then Exit;
MouseX := -1;
MouseY := -1;
ListBox1.Invalidate;
end;
procedure TForm1.ListBox1Click(Sender: TObject);
var
P: TPoint;
R: TRect;
Index: Integer;
begin
P := Point(MouseX, MouseY);
Index := ListBox1.ItemAtPos(P, True);
if (Index = -1) or (Index <> ListBox1.ItemIndex) then Exit;
R := ListBox1.ItemRect(Index);
R := Rect(R.Right-80, R.Top+4, R.Right-30, R.Top+24);
if not PtInRect(R, P) then Exit;
// click is on selected item's "button", do something...
end;