[un_pogaz]

What our friend is looking for is probably an anchorable "Edit ToC" window, like "Book View". Right ?
I'm sorry, but I don't think that's planned.

As said, the "Edit ToC" window (and the "Generate ToC" tool) is perfectly suited for this.
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Minute, In the "View" menu, there is a window "Table Of Contents" (anchorable) that allows you to preview and use the ToC. It is not editable, but allows you to navigate through the book.
Making sure you have it turned on, it can really help.

@KevinH: I'm not talking about adding a button, but replacing the action of the current ToC button.
It seems more intuitive than the ToC button, next to "Edit the metadata" is "Edit ToC", and not "Generate a new ToC that will overwrite the previous one".
Ok it's a change from the previous version, so, a otpion in the préférence? (This reminds me of the "Kobo Utilities" plugin in Caliber, it's possible to set the default action of the main button. A very nice option)