Author: Maarten de Haan

How to detect if a TreeView has a scrollbar?
How to change the scrollbar in a TreeView (if it has one)?

Answer:
1   
2   procedure TForm1.FormMouseWheelDown(Sender: TObject;
3     Shift: TShiftState;
4     MousePos: TPoint;
5     var Handled: Boolean);
6   
7   var
8     iMin, iMax: Integer;
9     bTreeViewVertScrollBarVisible: Boolean;
10  
11  begin
12    bTreeViewVertScrollBarVisible := True;
13    GetScrollRange(Form1.TreeView1.Handle, SB_VERT, iMin, iMax);
14    if iMin = iMax then
15      bTreeViewVertScrollBarVisible := False; // No scrollbar visible
16  
17    if bTreeViewVertScrollBarVisible then
18    begin
19      iPos := GetScrollPos(Form1.TreeView1.Handle, SB_VERT);
20      SetScrollPos(Form1.TreeView1.Handle, SB_VERT, iPos + 1, True);
21      // Don't set Handled to True!
22      // If you do that then ONLY the scrollbar changes but NOT the
23      // content of the TreeView!
24    end;
25  
26  end;