In my case I've just added some meta tag to force IE to render in IE8 compatible mode (when in edit mode or Composer on page edit):

protected override void OnInit(EventArgs e)
{
    base.OnInit(e);            

    //It's important that _extensionHandler initialized after base.OnInit.
    _extensionHandler.Initialize(this);

    if (_extensionHandler.ViewMode == ExtensionGeneric.ViewMode.ExtensionEditOnPageMode || _extensionHandler.ViewMode == ExtensionGeneric.ViewMode.ExtensionEditMode)
    {
        HtmlMeta httpEquiv = new HtmlMeta();
        httpEquiv.HttpEquiv = "X-UA-Compatible";
        httpEquiv.Content = "IE=EmulateIE7";
        Header.Controls.Add(httpEquiv);                        
    }
}

Hope this helps.

Frederik