Most people with dementia reach a stage called shadowing. It comes when their short-term memory decreases to a very short span.
You may tell her that you are just going in the kitchen to cook dinner, but from her point of view, after a couple of minutes she has forgotten; she is on her own and has no idea how long it has been like this. She doesnt know if its been 2 minutes, or 2 weeks; she doent know if there is anyone else around, she doesnt know if anyone is coming back or whether she has been abandoned; she is not sure about where she is, why she is there or what she is supposed to be doing and she is afraid. So she goes looking for you. This is happening in her mind, constantly, 24/7, every minute of her waking life she wants you within sight so that she is reassured and directed.
From your view point this is untenable. Once the lock down is over, it would be a good idea to get someone in to sit with her while you do other things. People like the Red Cross and Age UK will provide this service, though I think you may have to pay for it. You dont mention carers - if you dont have carers in I would reccommend them. OH has a carer in to help him wash/shower and dress etc in the mornings and it frees up my time and gives me an hour to get things done which I cant do at other times. You may also find that day care will provide her with company and give you a break.