參考：StackOverFlow、Control.Validating 事件

註意 TreeView 的 HideSelection 要保持為默認值True。

private TreeNode previousSelectedNode;

private void treeView1_Validating(object sender, System.ComponentModel.CancelEventArgs e)
{
    treeView1.SelectedNode.BackColor = SystemColors.Highlight;
    treeView1.SelectedNode.ForeColor = Color.White;
    previousSelectedNode = treeView1.SelectedNode;
}

 
private void treeView1_AfterSelect(object sender, TreeViewEventArgs e)
{
    if(previousSelectedNode != null)
    {
        previousSelectedNode.BackColor = treeView1.BackColor;
        previousSelectedNode.ForeColor = treeView1.ForeColor;
    }
}

Treeview 丟失焦點後依然高亮 SelectedNode