Excel Ribbon comboBox: How to set up a getSelectedItemIndex Callback

This is a follow-up question to this one:

Excel Ribbon comboBox: always highlight selection

The following code allows you to select from three paper sizes using a comboBox. There is only only issue: the selected item doesn't stay highlighted.

@OlleSjögren writes, that you need the getSelectedItemIndex-Callback to make this work.

Question: How do you build the getSelectedItemIndex-Callback?

If I add it to the XML, I get an error message: "the attribute is not declared".

Thanks!

' -- XML

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="LoadRibbon">
    <ribbon>
        <tabs>
            <tab id="Tabv3.1" label="TOOLS" insertAfterMso="TabHome">                                             
                <group id="GroupDemo2" 
                label="SelectPapersize"
                imageMso="AddInManager">
                    <comboBox id="ComboBox001"
                    label="comboBox001"
                    getText="ComboBox001_GetText"
                    onChange="ComboBox001_OnChange">
                        <item id="Item_A3"
                        label="A3"/>
                        <item id="Item_A4"
                        label="A4"/>
                        <item id="Item_A5"
                        label="A5"/>
                    </comboBox>
                </group>   
            </tab>
        </tabs>
    </ribbon>
</customUI>


' -- Callback VBA in Module "RibbonCallbacks"

Option Explicit
Public RibUI As IRibbonUI
Public Const myApp As String = "RibbApp", mySett As String = "Settings", myVal As String = "Value"

Sub LoadRibbon(Ribbon As IRibbonUI)
    Set RibUI = Ribbon
    RibUI.InvalidateControl "ComboBox001"
End Sub

'Callback for ComboBox001 onChange
Sub ComboBox001_OnChange(control As IRibbonControl, id As String)
    Select Case id
        Case "A3"
             ActiveSheet.PageSetup.PaperSize = xlPaperA3
        Case "A4"
            ActiveSheet.PageSetup.PaperSize = xlPaperA4
        Case "A5"
            ActiveSheet.PageSetup.PaperSize = xlPaperA5
    End Select
    RibUI.InvalidateControl "ComboBox001"
    SaveSetting myApp, mySett, myVal, id
End Sub

'Callback for ComboBox001 getText
Sub ComboBox001_getText(control As IRibbonControl, ByRef returnedVal)
  Dim comboVal As String
    comboVal = GetSetting(myApp, mySett, myVal, "No Value") 
    If comboVal <> "No Value" Then
      returnedVal = comboVal
    End If
End Sub


' -- VBA in "ThisWorkbook"

Private Sub Workbook_Open()

End Sub

Private Sub Workbook_SheetActivate(ByVal Sh As Object)
    Dim idx As String
    Select Case PaperSize
    Case "xlPaperA3"
        idx = "A3"
    Case "xlPaperA4"
        idx = "A4"
    Case "xlPaperA5"
        idx = "A5"
    End Select
    SaveSetting myApp, mySett, myVal, idx
    RibUI.InvalidateControl "ComboBox001"
End Sub

• Call back getSelectedItemIndex is available for dropDown instead of comboBox.
• Both XML and callback subs changed for dropDown.
• Note: the solution is only tested on Win11+O365.

' --- XML

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="LoadRibbon">
    <ribbon>
        <tabs>
            <tab id="Tabv3.1" label="TOOLS" insertAfterMso="TabHome">                                             
                <group id="GroupDemo2" 
                label="SelectPapersize"
                imageMso="AddInManager">
                    <dropDown id="DropDown1"
                    label="DropDown1"
                    onAction="DropDown1_onAction"
                    getSelectedItemIndex="DropDown1_GetSelectedItemIndex"
                    >
                        <item id="Item_A3"
                        label="A3"/>
                        <item id="Item_A4"
                        label="A4"/>
                        <item id="Item_A5"
                        label="A5"/>
                    </dropDown>
                </group>   
            </tab>
        </tabs>
    </ribbon>
</customUI>

' -- Stardard module
Option Explicit
Public RibUI As IRibbonUI

Sub LoadRibbon(ribbon As IRibbonUI)
    Set RibUI = ribbon
    RibUI.InvalidateControl "DropDown1"
End Sub

'Callback for DropDown1 onAction
Sub DropDown1_onAction(control As IRibbonControl, id As String, index As Integer)
    Dim iSize As Long
    Select Case Right(id, 2)
        Case "A3"
             iSize = xlPaperA3
        Case "A4"
            iSize = xlPaperA4
        Case "A5"
            iSize = xlPaperA5
    End Select
    If iSize > 0 Then _
        ActiveSheet.PageSetup.PaperSize = iSize
End Sub

'Callback for DropDown1 getSelectedItemIndex
Sub DropDown1_GetSelectedItemIndex(control As IRibbonControl, ByRef returnedVal)
    returnedVal = GetPageSize
End Sub

Function GetPageSize() As String
        Select Case ActiveSheet.PageSetup.PaperSize
        Case xlPaperA3
            GetPageSize = 0 '"A3"
        Case xlPaperA4
            GetPageSize = 1 ' "A4"
        Case xlPaperA5
            GetPageSize = 2 ' "A5"
        End Select
End Function

' -- ThisWorkbook
Private Sub Workbook_SheetActivate(ByVal Sh As Object)
   RibUI.InvalidateControl "DropDown1"
End Sub