If @menu_label == NULL, we create a default page->menu_label. This took
@tab_label.get_label() and passed that to page->menu_label.set_text().
This is wrong because we set the plain text of the menu_label from the
rich text of @tab_label. So, if @tab_label used mnemonics or markup, our
menu_label got the raw underline or markup tags shown in it as raw text.

As we call set_text() on the menu Label, the fix is to be symmetric: use
@tab_label’s get_text() as source, as that strips underlines and markup.

It’s not worth making the default Label ‘inherit’ :use-underline/markup;
that’s a slippery slope, and users wanting such things can just create a
fully fledged GtkLabel to pass as @menu_label to suppress the default.

https://bugzilla.gnome.org/show_bug.cgi?id=705509