Based on my test using ChartSeries_DataPoint.aspx sample, using OnAfterInitialize client side event handler, we could get the FilterExpression element based on the available filter info. However, the element will only be available after the pop up has been displayed, hence the setInterval, as shown in the snippet: 

function WebGrid1_OnAfterIntialize(id) 
{
    var grid = ISGetObject(id);

    setInterval(function () {
        try {
            var filterInfo = grid.CachePivotFilterInfoCollection[0];
            var filterExpElem = test.GetFilterExpressionElement();

            filterExpElem.parentNode.parentNode.style.display = "none";
        }
        catch (err) {}
                
    }, 150);
            
}

I am very sorry that the attachment is corrupted, I have re-attached the sample. Thank you for the feedback, I will consult your suggestion to my supervisor.

In the sample, in the JS function executed after the ajax request, the function to show / hide the grid and menubar is executed. I also refresh the grid to resolve the maximize issue.

Casting to UXListBoxItem only works if the UXListBox is in Unbound mode. In your case, the UXListBox is bounded to a collection of object. In such case, you will need to cast the item to the Object of the collection.

Attached is a simple sample which demonstrate such scenario.

You could iterate the SelectedItems property in order to retrieve the selected items from the list box. In the unbound mode, each object will need to be cast to UXListBoxItem, here is the snippet:

private void uXButton1_Click(object sender, RoutedEventArgs e)
{
    string test = "";
    foreach (UXListBoxItem item in (IEnumerable<object>)uXListBox1.SelectedItems)
    {
        test += (string.IsNullOrEmpty(test) ? "" : ", ") + item.Content.ToString();
    }

    MessageBox.Show("Selected item: " + Environment.NewLine + test);
}

Based on the snippet you provided, you are using BatchUpdate feature and VirtualLoad paging. Under such scenario, you could set the Grid property HaltLoadMore set to true in order to abort the load more process once. In your case, you will need to set the property before invoking the Select or ActivateEdit method. Here is the snippet:

function AddRow()
{
    var grid = ISGetObject("WebGrid1");
    var newRow = grid.RootTable.NewRow(); // create new row object
    var cells = newRow.GetCells(); // get WebGridCell collection

    // populate new row object     
    .
    .
    .
    
    // insert new record
    newRow.AddPendingChanges();

    grid.HaltLoadMore = true
    newRow.GetCell(0).ActivateEdit();
    
}