Every node of a treeview is a TGadget, thus you can make usage of the SetGadgetExtra()/GadgetExtra() functions, which can attach an object to the TGadget (your nodes).!
Create only one! Textarea.
Attach your “textarea text” via SetGadgetExtra to the node you add to the treeview.
When you select a node later in the treeview, read the “textarea text” via GadgetExtra and present it in the real Textarea. Just don't forget to cast the object retrieved from the node, depending on what you have stored.
For text: MyString:String = String(GadgetExtra(TreeviewNode))
Create only one! Textarea.
Attach your “textarea text” via SetGadgetExtra to the node you add to the treeview.
When you select a node later in the treeview, read the “textarea text” via GadgetExtra and present it in the real Textarea. Just don't forget to cast the object retrieved from the node, depending on what you have stored.
For text: MyString:String = String(GadgetExtra(TreeviewNode))