Just created an SharePoint App project in Visual Studio 2019, create two ribbon buttons by this xml. Then back to classic mode, the ribbon tab “Items” will stop responding.
<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<CustomAction Id="35ae46bd-b38f-4ce1-a2e2-4db33bde3147.RibbonCustomAction2"
RegistrationType="List"
RegistrationId="100"
Location="CommandUI.Ribbon"
Sequence="10002"
Title="Invoke 'RibbonCustomAction2' action">
<CommandUIExtension>
<CommandUIDefinitions>
<CommandUIDefinition Location="Ribbon.ListItem.Actions.Controls._children">
<Controls Id="controls1">
<Button Id="Ribbon.ListItem.Actions.RibbonCustomAction2Button"
Alt="Request RibbonCustomAction2"
Sequence="100"
Command="Invoke_RibbonCustomAction2ButtonRequest"
LabelText="Quantr Logic 2"
TemplateAlias="o1"
Image32by32="https://www.quantr.hk/wp-content/uploads/2018/10/quantr_logo32.png"
Image16by16="https://www.quantr.hk/wp-content/uploads/2018/10/quantr_logo16.png" />
<Button Id="Ribbon.ListItem.Actions.RibbonCustomAction3Button"
Alt="Request RibbonCustomAction3"
Sequence="101"
Command="Invoke_RibbonCustomAction3ButtonRequest"
LabelText="Quantr 3"
TemplateAlias="o2"
Image32by32="https://www.quantr.hk/wp-content/uploads/2018/10/quantr_logo32.png"
Image16by16="https://www.quantr.hk/wp-content/uploads/2018/10/quantr_logo16.png" />
</Controls>
</CommandUIDefinition>
</CommandUIDefinitions>
<CommandUIHandlers>
<CommandUIHandler Command="Invoke_RibbonCustomAction2ButtonRequest"
CommandAction="~appWebUrl/Pages/Default.aspx?{StandardTokens}&SPListItemId={SelectedItemId}&SPListId={SelectedListId}"/>
<CommandUIHandler Command="Invoke_RibbonCustomAction3ButtonRequest"
CommandAction="~appWebUrl/Pages/Default.aspx?{StandardTokens}&SPListItemId={SelectedItemId}&SPListId={SelectedListId}"/>
</CommandUIHandlers>
</CommandUIExtension >
</CustomAction>
</Elements>