Register a page that is done building with its manager
Every page's constructor ends with this, and none registers before: the manager hands setting_changed () every "changed" from the moment of the register, and a page still being built takes one with its row pointers unset — straight into a setter of its own, which, unlike the entry path, has no tolerance for a null row. Nothing but building a page not iterating the main loop keeps one from landing in between, which is not a page's to guarantee. The page comes back so that a constructor can end with the register itself: one place holds the rule, rather than a copy of it per page.
| this |
a PreferencesPage instance |
| manager |
a PreferencesManager instance |
|
this, as the Widget a page constructor returns |