The CollectionEditor control has a resize functionality which allows you to drag its bottom right cornet, which changes the Width and Height of the root element in the ControlTemplate of the CollectionEditor. When hosted in CollectionEditorPicker, the CollectionEditor is placed in a RadDropDownButton's DropDownContent which is basically a Popup with Placement=Bottom. When the popup goes near the bottom edge of the screen, thus doesn't having enough height to render all its contents, the Popup is automatically re-positioned so it aligns top to its parent element (the RadDropDownButton in this case). In other words, the Popup starts behaving as if its Placement=Top. 

This behavior leads to issues with the vertical resizing of the CollectionEditor control. Firstly, when the Popup is automatically aligned using the Top placement, if you resize to a size small enough to fit under the picker control, the Popup gets re-positioned below the picker. Secondly, if you vertically resize the CollectionEditor to a bigger size while aligned to Top, the resizing action will increase the control height, but because of the alignment it will look like the resizing happens from the top corner of the control. In this scenario, also the buttons of the CollectionEditor disappear.

To work this around, you can disable the resizing behavior of the CollectionEditor control.

 private void RadSyntaxEditor_PreviewKeyDown(object sender, System.Windows.Input.KeyEventArgs e)
 {
     if (KeyboardModifiers.IsControlDown && e.Key == Key.Space)
     {		
        e.Handled = true;

		var editor = (RadSyntaxEditor)sender;
        CaretPosition startPosition = new CaretPosition(editor.CaretPosition);
        CaretPosition endPosition = new CaretPosition(editor.CaretPosition);
        editor.IntelliPrompts.CompletionListWindow.Show(startPosition, endPosition);
		editor.IntelliPrompts.CompletionListWindow.HorizontalOffset = 0;
		editor.IntelliPrompts.CompletionListWindow.VerticalOffset = 0;
     }
 }

 private void RadSyntaxEditor_Loaded(object sender, System.Windows.RoutedEventArgs e)
 {
    var editor = (RadSyntaxEditor)sender;
    var caret = editor.FindChildByType<Telerik.Windows.Controls.SyntaxEditor.UI.Caret>();
    editor.IntelliPrompts.CompletionListWindow.PlacementTarget = caret;
    editor.IntelliPrompts.CompletionListWindow.Placement = System.Windows.Controls.Primitives.PlacementMode.Bottom;
 }

System.ArgumentException: 'Requested distance is outside the content of the associated document.'PresentationFramework.dll!System.Windows.Documents.TextPointer.TextPointer(System.Windows.Documents.TextContainer textContainer, int offset, System.Windows.Documents.LogicalDirection direction)
  PresentationFramework.dll!System.Windows.Documents.TextContainer.CreatePointerAtOffset(int offset, System.Windows.Documents.LogicalDirection direction) 
  PresentationFramework.dll!System.Windows.Documents.TextParentUndoUnit.Do() 
  PresentationFramework.dll!MS.Internal.Documents.UndoManager.Redo(int count) 
  PresentationFramework.dll!System.Windows.Documents.TextStore.RedoQuietly(int count)  PresentationFramework.dll!System.Windows.Documents.TextStore.SetFinalDocumentState(MS.Internal.Documents.UndoManager undoManager, System.Collections.Stack imeChangeStack, int appChangeCount, int imeSelectionAnchorOffset, int imeSelectionMovingOffset, int appSelectionAnchorOffset, int appSelectionMovingOffset)  PresentationFramework.dll!System.Windows.Documents.TextStore.HandleCompositionEvents(int previousUndoCount)  PresentationFramework.dll!System.Windows.Documents.TextStore.GrantLockWorker(MS.Win32.UnsafeNativeMethods.LockFlags flags)   PresentationFramework.dll!System.Windows.Documents.TextStore.RequestLock(MS.Win32.UnsafeNativeMethods.LockFlags flags, out int hrSession)

dialog.Filter = "Special files|.*_FileNamePart.txt"

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.dateRangePicker.Loaded += DateRangePicker_Loaded;
    }

    private void DateRangePicker_Loaded(object sender, RoutedEventArgs e)
    {
        DateRangeMaskedInput dateRangeMaskedInput = this.dateRangePicker.FindChildByType<DateRangeMaskedInput>();

        if (dateRangeMaskedInput != null)
        {
            dateRangeMaskedInput.ValueChanging += DateRangeMaskedInput_ValueChanging;
        }
    }

    private void DateRangeMaskedInput_ValueChanging(object? sender, Telerik.Windows.Controls.MaskedInput.RadMaskedInputValueChangingEventArgs e)
    {
        if (e.NewValue.ToString().Any(char.IsLetter))
        {
            e.Handled = true;
        }
    }
}