Handling tab presses in UWP TextBoxes

// Replace default "tab means tab to next control" action.
// Note: This isn't even an alpha; I'm fiddling with this problem now.
case VirtualKey.Tab:
    if (Window.Current.CoreWindow.GetKeyState(VirtualKey.Shift).HasFlag(CoreVirtualKeyStates.Down))
    {
        StringBuilder builder = new StringBuilder();
        string[] astrLines = this.SelectedText.Split(this.SelectionLineEnding); // I'm doing lots of xplat stuff. SelectionLineEnding in a UWP TextBox is \r
        string toAppend = string.Empty;

        if (astrLines.Length > 0 && astrLines[astrLines.Length - 1].EndsWith(this.SelectionLineEnding))
        {
            astrLines[astrLines.Length - 1] = astrLines[astrLines.Length - 1].RemoveLastNewLine(true); // In this case, blasts last \r if it exists
            toAppend = this.SelectionLineEnding;
        }

        foreach (string line in astrLines)
        {
            // See below for _deleteATab...
            builder.Append(_deleteATabWorthOfLeadingSpaces(line) + this.SelectionLineEnding);
        }

        this.SelectedText = builder.ToString().RemoveLastNewLine(true) + toAppend;
    }
    else
    {
        // MdTextBox is an extension of TextBox, and Tab is just a string of spaces that's whatever the user says Tab should be (default "    ")
        this.SelectedText = MdTextBox.Tab + this.SelectedText.Replace(this.SelectionLineEnding, this.SelectionLineEnding + MdTextBox.Tab);
        if (this.SelectedText.EndsWith(this.SelectionLineEnding))
        {
            this.SelectedText = this.SelectedText.DeleteLastNChars(MdTextBox.Tab.Length);   // Does what it seems to; Removes X characters from the end of the string.
        }
    }

    e.Handled = true;
    break;

// ...

private string _deleteATabWorthOfLeadingSpaces(string value)
{
    string ret = value;
    int intTabSize = MdTextBox.Tab.Length;
    Tuple<string, string> tupe = value.PullLeadingAndTrailingSpaces(); // Tuple will have Item1 with spaces at front & Item2 with spaces at end

    if (tupe.Item1.Length >= intTabSize)
    {
        ret = value.Substring(intTabSize);
    }
    else if (tupe.Item1.Length > 0)
    {
        ret = value.Substring(tupe.Item1.Length);
    }

    return ret;
}