Delphi – expand/collapse TreeView only by node image click

Add BeginUpdate and EndUpdate as follows and it should work:

procedure TfrmMain.UpdateGroupImages;
var
  Node: TTreeNode;
  i: Integer;
begin
  TreeView1.BeginUpdate;
  try
    if TreeView1.Items.Count > 0 then
    begin
      for i := 0 to TreeView1.Items.Count - 1 do
      begin
        Node := TreeView1.Items[i];

        case Node.ImageIndex of
          imgGroupClosed, imgGroupOpen:
            begin
              if Node.HasChildren then
              begin
                if Node.Expanded then
                begin
                  Node.ImageIndex := imgGroupOpen;
                  Node.SelectedIndex := imgGroupOpen;
                end
                else
                begin
                  Node.ImageIndex := imgGroupClosed;
                  Node.SelectedIndex := imgGroupClosed;
                end
              end
              else
              begin
                Node.ImageIndex := imgGroupClosed;
                Node.SelectedIndex := imgGroupClosed;
              end;
            end;
        end;
      end;
    end;
  finally
    TreeView1.EndUpdate;
  end;
end;

Detect a click or double-click on the control in general with the OnClick and OnDblClick events. To find out which specific node was clicked, use the control's GetNodeAt method with the mouse's current coordinates.

You can get the coordinates as they were at the time of the click with GetMessagePos; reading Mouse.CursorPos will give you the coordinates at the time you read the property, which might be very different from the time of the click if there is any sort of load on the system, and that can lead to detecting clicks at places that aren't even within the bounds of your control.