Use the following code snippet:
Sub New()
InitializeComponent()
Me.RadCheckedDropDownList1.ShowCheckAllItems = True
Me.RadCheckedDropDownList1.AutoSizeItems = True
For x = 1 To 20
Dim i As New DescriptionTextCheckedListDataItem
i.Value = x
i.Text = x.ToString()
i.DescriptionText = "abc def ghijkl mnop abc def ghijkl mnop" & x
Me.RadCheckedDropDownList1.Items.Add(i)
Next
End Sub
Open the popup, scroll to the bottom and then back to the top.
Expected: The CheckAllItem is visible
Actual: The CheckAllItem is hidden