1) SizeNotifiable, defined as the following:

    class SizeNotifiable<RegisterBehaviour, true> : private RegisterBehaviour, public SizeNotifiableBase { ... };

gets constructed - it inherits from SizeNotifiableBase which declares NotifyResChanged() as virtual. It also inherits from RegisterBehaviour, which must provide a Register() method.

2) In SizeNotifiable's constructor RegisterBehaviour::Register(this) gets called. Being in the constructor, virtual calls on this are UB.

3) When RegisterBehaviour == AutoRegister, we have Register defined as the following:

    void AutoRegister::Register (SizeNotifiableBase* parNotifiable) {
        m_id = m_sdlmain->RegisterForResChange(parNotifiable);
    }

note that parNotifiable is received from SizeNotifiable's constructor, so it's a partially constructed object.

4) RegisterForResChange() is defined as:

    size_t SDLMain::RegisterForResChange (SizeNotifiableBase* parNotif) {
        parNotif->NotifyResChanged(m_localData->sizeratio);
        return m_localData->resChangeNotifList.Add(parNotif);
    }

parNotif is, again, the partly constructed object pointed by this in SizeNotifiable's constructor. NotifyResChanged() is a virtual method, thus the bug.
Objects being registered need to be notified straight away since they have no clue about the current size held by SDLMain.