Virtual StringTree: Text returned by the OnGetHint event handler is not shown in the hint window

Question

I wrote a handler for the OnGetHint event of the Virtual StringTree control, and I'm sure that the hint text is returned, but the hint window is blank, see the screenshot below, what might caused this problem? thank you in advance.

The settings of the control:

AnimationDuration := 0;
ClipboardFormats.Clear;
ClipboardFormats.Add('HTML Format');
ClipboardFormats.Add('Plain text');
ClipboardFormats.Add('Rich Text Format');
ClipboardFormats.Add('Rich Text Format Without Objects');
ClipboardFormats.Add('Unicode text');
ClipboardFormats.Add('Virtual Tree Data');
DefaultPasteMode := amInsertBefore;
DragOperations := [doCopy, doMove, doLink];
EditDelay := 200;
HintMode := hmHint;
Images := dmImages.lstProjectTree;
IncrementalSearch := isAll;
Indent := 16;
Margin := 0;
ParentFont := False;
PopupMenu := pmTree;
SelectionCurveRadius := 5;
OnCollapsed := treePrjCollapsed;
OnColumnClick := treePrjColumnClick;
OnDblClick := treePrjDblClick;
OnDragAllowed := treePrjDragAllowed;
OnDragOver := treePrjDragOver;
OnDragDrop := treePrjDragDrop;
OnExpanded := treePrjExpanded;
OnFocusChanged := treePrjFocusChanged;
OnGetCursor := treePrjGetCursor;
OnGetText := treePrjGetText;
OnPaintText := treePrjPaintText;
OnGetImageIndexEx := treePrjGetImageIndexEx;
OnGetHint := treePrjGetHint;
OnKeyDown := treePrjKeyDown;
OnMouseDown := treePrjMouseDown;
OnNewText := treePrjNewText;
OnNodeMoving := treePrjNodeMoving;
with Columns.Add do begin
  Position := 0;
  Width := 153;
end;
with Columns.Add do begin
  Layout := blGlyphRight;
  MinWidth := 20;
  Position := 1;
  Width := 20;
end;
with Columns.Add do begin
  Layout := blGlyphRight;
  MinWidth := 20;
  Position := 2;
  Width := 20;
end;
with Columns.Add do begin
  Layout := blGlyphRight;
  MinWidth := 16;
  Position := 3;
  Width := 20;
end;

Answer 1

I found the culprit of the problem myself! It's AnimationDuration := 0

As Rob suggested, I added this finding as the answer so that I can accept it.