Scroll TTreeView while dragging over/near the edges

Question

I have a TTreeView that can have lots of nodes, when a lot of nodes are expanded the tree uses a lot of screen space.

Now suppose I want to drag a node that is near

Answer 1

Here's an alternative based on the fact that the selected node always automatically scrolls in view.

type
  TForm1 = class(TForm)
    TreeView1: TTreeView;
    TreeView2: TTreeView;
    procedure TreeViewDragOver(Sender, Source: TObject; X, Y: Integer;
      State: TDragState; var Accept: Boolean);
    procedure TreeViewEndDrag(Sender, Target: TObject; X, Y: Integer);
    procedure TreeViewMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
  private
    FDragNode: TTreeNode;
    FNodeHeight: Integer;
  end;

...

procedure TForm1.TreeViewMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  with TTreeView(Sender) do
  begin
    FDragNode := GetNodeAt(X, Y);
    if FDragNode <> nil then
    begin
      Selected := FDragNode;
      with FDragNode.DisplayRect(False) do
        FNodeHeight := Bottom - Top;
      BeginDrag(False, Mouse.DragThreshold);
    end;
  end;
end;

procedure TForm1.TreeViewDragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
var
  Pt: TPoint;
  DropNode: TTreeNode;
begin
  Accept := Source is TTreeView;
  if Accept then
    with TTreeView(Source) do
    begin
      if Sender <> Source then
        Pt := ScreenToClient(Mouse.CursorPos)
      else
        Pt := Point(X, Y);
      if Pt.Y < FNodeHeight then
        DropNode := Selected.GetPrevVisible
      else if Pt.Y > (ClientHeight - FNodeHeight) then
        DropNode := Selected.GetNextVisible
      else
        DropNode := GetNodeAt(Pt.X, Pt.Y);
      if DropNode <> nil then
        Selected := DropNode;
    end;
end;

procedure TForm1.TreeViewEndDrag(Sender, Target: TObject; X, Y: Integer);
var
  DropNode: TTreeNode;
begin
  with TTreeView(Sender) do
    if Target <> nil then
    begin
      DropNode := Selected;
      DropNode := Items.Insert(DropNode, '');
      DropNode.Assign(FDragNode);
      Selected := DropNode;
      Items.Delete(FDragNode);
    end
    else
      Selected := FDragNode;
end;

You may want to link the OnDragOver event handler to the parent of the TreeView too, which results in scrolling ánd dropping when the mouse is outside the TreeView. If you dó want the scrolling, but not the dropping when the mouse is outside the TreeView, then check if Target = Sender in the OnEndDrag event handler.