TreeNode Selected BackColor while TreeView not focused

1. Set the TreeView.DrawMode to TreeViewDrawMode.OwnerDrawText:

   treeView.DrawMode = TreeViewDrawMode.OwnerDrawText;

2. Set the Treview.HideSelection to false, so that the node states will be kept as selected:

   treeView.HideSelection= false;

3. Add DrawNode event handler to draw the background using SystemColors.Highlight color:

   private void treeView_DrawNode(object sender, DrawTreeNodeEventArgs e)
   {
     if (e.Node == null) return;
   
     // if treeview's HideSelection property is "True", 
     // this will always returns "False" on unfocused treeview
     var selected = (e.State & TreeNodeStates.Selected) == TreeNodeStates.Selected;
     var unfocused = !e.Node.TreeView.Focused;
   
     // we need to do owner drawing only on a selected node
     // and when the treeview is unfocused, else let the OS do it for us
     if (selected && unfocused)
     {
       var font = e.Node.NodeFont ?? e.Node.TreeView.Font;
       e.Graphics.FillRectangle(SystemBrushes.Highlight, e.Bounds);
       TextRenderer.DrawText(e.Graphics, e.Node.Text, font, e.Bounds, SystemColors.HighlightText, TextFormatFlags.GlyphOverhangPadding);
     }
     else
     {
       e.DrawDefault = true;
     }
   }