How can I prevent that users accidentally close an element in the tab bar? (TOC View with Form)

Hi community,

I right now work on a MES scenario were I use a TOC View as interface for end users. For some specific use cases (parallel work) I allow that user use multiple instances of the same TOC View in parallel. 

As user may work with a couple of these tabs at the same time, I want to prevent that they accidentally close any of the tabs. They of course can open the TOC View again and continue were they stopped. But it would add a some comfort if a warning message appears where users can decide if they really want to close the tab.

I tried to use an "onBeforeUnload" Method on my Form that returns "true"  or "false". The "false" is some kind of accepted by Innovator, as it will then throw me the "Item not saved" error message. But it will still close the tab independent from the user selection.

Does anyone know a simple solution to prevent closing a tab?

Every idea is welcomed!! :-)