I tried to reproduce the problem by modifying SimpleLayout.aspx sample file of WebPaneManager. An unbound WebTreeView is added inside the ContentTemplate of "Side" WebPane.

<ISWebTreeView:WebTreeView ID="WebTreeView1" runat="server" Height="300px" Width="250px"
    ExpandedDepth="-1" RenderingMode="HTML5">
    <Nodes>
        <ISWebTreeView:WebTreeViewNode Name="WebTreeViewNode0" Text="Personal Folders"
            Expanded="True">
            <ISWebTreeView:WebTreeViewNode Name="DeletedItems" Text="Deleted Items">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="Drafts" Text="Drafts">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="Inbox" Text="Inbox">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="JunkE-mail" Text="Junk E-mail">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="Outbox" Text="Outbox">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="RSSFeeds" Text="RSS Feeds">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="SentItems" Text="Sent Items">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="SearchFolders" Text="Search Folders">
                <ISWebTreeView:WebTreeViewNode Name="CategorizedMail" Text="Categorized Mail">
                </ISWebTreeView:WebTreeViewNode>
                <ISWebTreeView:WebTreeViewNode Name="ForFollowUp" Text="For Follow Up">
                </ISWebTreeView:WebTreeViewNode>
                <ISWebTreeView:WebTreeViewNode Name="LargeMail" Text="Large Mail">
                </ISWebTreeView:WebTreeViewNode>
                <ISWebTreeView:WebTreeViewNode Name="UnreadMail" Text="Unread Mail">
                </ISWebTreeView:WebTreeViewNode>
            </ISWebTreeView:WebTreeViewNode>
        </ISWebTreeView:WebTreeViewNode>
        <ISWebTreeView:WebTreeViewNode Name="WebTreeViewNode1" Text="Archive Folder">
            <ISWebTreeView:WebTreeViewNode Name="DeletedItems2" Text="Deleted Items">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="SentItems" Text="Sent Items">
            </ISWebTreeView:WebTreeViewNode>
            <ISWebTreeView:WebTreeViewNode Name="SearchFolders2" Text="Search Folders">
            </ISWebTreeView:WebTreeViewNode>
        </ISWebTreeView:WebTreeViewNode>
    </Nodes>
    <NodeSettings EnableAnimation="True" />
</ISWebTreeView:WebTreeView>

To become more similar to your WebPane settings, the properties/attributes of "Side" pane is set to:

<ISWebDesktop:WebPane Name="Side" Text="Side" Width="Custom" WidthValue="15%"
    HeaderVisible="Yes" AllowCollapse="No" HeightValue="70%" Height="Custom" ContentScrollable="false">
    <ContentTemplate>
        <ISWebTreeView:WebTreeView ID="WebTreeView1" runat="server" Height="300px" Width="250px"
            ExpandedDepth="-1" RenderingMode="HTML5">
            ...
            <NodeSettings EnableAnimation="True" />
        </ISWebTreeView:WebTreeView>
    </ContentTemplate>
</ISWebDesktop:WebPane>

Save the changes and view the modified page in IE 11 browser. The reported problem doesn't persist. User can select node; WebTreeViewNode becomes underlined when user hovering mouse pointer over a node.

I enclose the modified sample for your reference. Please feel free to let me know if you find anything that I might miss during my attempt to reproduce the reported problem.

Hi and thank you for your response.
I found the problem but I don't understand why the change is needed.
I had to add this into the header section of the html file.

     <style type="text/css">        html, body, form
        {
            height: 100%;
            width: 100%;
            margin: 0px;
            padding: 0px;
        }
    </style>

But I already had this code in a CSS flie and included that in the header section such
<link href="~/Css/mainStyle.css" rel="stylesheet" type="text/css" />
But if I do it that way the tree-view failure hapens when browsing in IE11.
To be sure I have even tried to put nothing else than this needed style into the css file. But even then the problem still happen.

mainStyle.css:

html, body, form
{
   height: 100%;
   width: 100%;
   margin: 0px;
   padding: 0px;
}

TJ

You can try to use DOM Inspector to find out if there is any css declaration which may override the style for html, body, and form.

However, glad to hear the good news.

Should you need further assistance or run into any problems regarding our controls, feel free to post it into our community. We would be happy to assist you again.