how to manage invalidate between two tvirtualstringtree in same form?

i have two tvirtualstringtree in a form i wanted to invalidate them using timers

but i stuck with problems it seems that only 1 VST got invalidated and the other one ignored any idear why ?

i use invalidate with current code within timer

procedure Tform1.Timerval1Timer(Sender: TObject);
{$J+}
Const
iCount: Cardinal = 0;
{$J-}
begin
Inc(iCount);
if (iCount * TTimer(Sender).Interval) > 500 then
begin
xbool := NOT xbool;
iCount := 0;
end;
VDT1.Invalidate;
end;

procedure Tform1.Timerval2Timer(Sender: TObject);
{$J+}
Const
iCount: Cardinal = 0;
{$J-}
begin
Inc(iCount);
if (iCount * TTimer(Sender).Interval) > 500 then
begin
xbool := NOT xbool;
iCount := 0;
end;
VDT2.Invalidate;
end;

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issue happens when i use beforecellpaint on each VSt

procedure Tform1.VDT1BeforeCellPaint(Sender: TBaseVirtualTree;
  TargetCanvas: TCanvas; Node: PVirtualNode; Column: TColumnIndex;
  CellPaintMode: TVTCellPaintMode; CellRect: TRect; var ContentRect: TRect);
var
Data: PUserdataon;
NewRect: TRect;
xOff, yOff: Integer;

begin
GradientFillCanvas(TargetCanvas, clred, clWhite, CellRect, gdHorizontal); // GraphUtil

if not Assigned(Node) then
begin
exit;
end;

Data := vdt1.GetNodeData(Node);

begin
NewRect := CellRect;
NewRect.Left := NewRect.Left + 2;
NewRect.Width := 24;
NewRect.Top := NewRect.Top;
NewRect.Bottom := NewRect.Bottom;
xOff := CellRect.Left + 70;
yOff := CellRect.Top +((30 - TargetCanvas.TextHeight('H')) div 2);
if xbool or (Data.FObject.bool= false) then
TargetCanvas.font.color := clred
else
TargetCanvas.font.color := clgray;
TargetCanvas.TextOut(xOff, yOff, Data.FObject.Username);

end;

end;

//vdt2 paint

procedure Tform1.VDt2BeforeCellPaint(Sender: TBaseVirtualTree;
  TargetCanvas: TCanvas; Node: PVirtualNode; Column: TColumnIndex;
  CellPaintMode: TVTCellPaintMode; CellRect: TRect; var ContentRect: TRect);
var
Data: PUserdataoff;
NewRect: TRect;
xOff, yOff: Integer;

begin
GradientFillCanvas(TargetCanvas, clred, clWhite, CellRect, gdHorizontal); // GraphUtil

if not Assigned(Node) then
begin
exit;
end;

Data := vdt2.GetNodeData(Node);

begin
NewRect := CellRect;
NewRect.Left := NewRect.Left + 2;
NewRect.Width := 24;
NewRect.Top := NewRect.Top;
NewRect.Bottom := NewRect.Bottom;
xOff := CellRect.Left + 70;
yOff := CellRect.Top +((30 - TargetCanvas.TextHeight('H')) div 2);
if xbool or (Data.FObject.bool= false) then
TargetCanvas.font.color := clred
else
TargetCanvas.font.color := clgray;
TargetCanvas.TextOut(xOff, yOff, Data.FObject.Username);

end;

end; 

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To be mor spicific issue happen when i try to draw blink text with

if xbool or (Data.FObject.bool= false) then

Select allOpen in new window

is my timer for blink text has issue ?