This is not necessarily a poorly indexed array, the key could be representing an id, and thus be perfectly legitimate. This main source of the problem seems to be that readPropertiesUntil is creating indices while it is simply trying to read. This is inconsistent with PHP's behaviour.

However, getValue does not have its argument $objectOrArray passed as a reference, the original array should not be changed. You should only see the problem arise if you are using objects.

Plus, this whole concept seems rather inefficient. It means that if I ask for index 1000, it will create a thousand empty items just to later call the last item and trash the rest.

You're right, a poorly indexed array isn't the best way to describe it, I was strapped for time and couldn't think of a way of describing them concisely (I guess non-consecutive would be better).

I agree it seems like odd behaviour, and yes because we're using Doctrine it is actually performing this on a PersistentCollection and hence it is actually creating these empty indexes.

I don't think it wouldn't even get to index 1,000 - it seems to limit the amount of properties it will read based on the amount of properties in the array. So it would iterate once, try and access index 0, fail and create that index, and then stop. When I get time I will put together some tests to try this theory.

Then when calling PropertyAccessor::getValue, it tries to iterate through all the properties of 'main' as if it was a zero indexed array.

This is not true. PropertyAccessor only accessed/creates array indexes that are explicitly requested.
So if the zero-indexes element is requested, then by the form component. Maybe by the resizing logic of the collection. So the fault is definitely not in the PropertyAccessor which makes the ticket confusing. Please create a new ticket if you can provide reproducible code. Then we can look where the error actually lies.

Btw, seems related to #7828