Found another way to hangs the SharePoint Online
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>