add a method to determine the top node of an thereby invisible subtree

- added method 'TBaseVirtualTree.GetTopInvisibleParent' to retrieve the topmost node of an effectively invisible subtree
- the resulting node can be invisible by lacking the 'vsVisible'-flag on its own or having its parent lacking the 'vsExpanded' flag
- all of its sub-nodes can be considered effectively invisible
- all of its ancestor-nodes can be considered to be effectively visible
- 'nil' is returned if all nodes from 'Node' up to 'FRoot' are effectively visible

Author: René Hoffmann

Source/VirtualTrees.pas

@@ -3138,6 +3138,7 @@   TBaseVirtualTree = class(TCustomControl)
       var Text: string); virtual;
     function GetTreeRect: TRect;
     function GetVisibleParent(Node: PVirtualNode; IncludeFiltered: Boolean = False): PVirtualNode;
+    function GetTopInvisibleParent(Node: PVirtualNode): PVirtualNode;
     function HasAsParent(Node, PotentialParent: PVirtualNode): Boolean;
     function InsertNode(Node: PVirtualNode; Mode: TVTNodeAttachMode; UserData: Pointer = nil): PVirtualNode;
     procedure InvalidateChildren(Node: PVirtualNode; Recursive: Boolean);
@@ -29908,6 +29909,31 @@ function TBaseVirtualTree.GetVisibleParent(Node: PVirtualNode; IncludeFiltered:
 
 //----------------------------------------------------------------------------------------------------------------------
 
+function TBaseVirtualTree.GetTopInvisibleParent(Node: PVirtualNode): PVirtualNode;
+
+// Returns the last (furthest) parent node of Node which is invisible.
+
+var
+  Run: PVirtualNode;
+
+begin
+  Assert(Assigned(Node), 'Node must not be nil.');
+  Assert(Node <> FRoot, 'Node must not be the hidden root node.');
+
+  Result := nil;
+
+  Run := Node.Parent;
+  while (Run <> FRoot) do
+  begin
+    if not ( (vsVisible in Run.States) and (vsExpanded in Run.Parent.States) ) then
+      Result := Run;
+    Run := Run.Parent;
+  end;
+
+end;
+
+//----------------------------------------------------------------------------------------------------------------------
+
 function TBaseVirtualTree.HasAsParent(Node, PotentialParent: PVirtualNode): Boolean;
 
 // Determines whether Node has got PotentialParent as one of its parents.