...expects Lizzy to accept his proposal; at this stage we know only that Lizzy thinks he is confident of her acceptance.

As he said this, she could easily see that he had no doubt of a favourable answer. (Ch. 34)

At the same time, I do think Darcy had reason to think Lizzy would be receptive. In addition to her arch manner in most conversation, she had told him where to find her walking alone at Rosings. Of course Lizzy meant that he should leave her alone because that's what she wanted, but Darcy seems to have misinterpreted it as a subtle invitation to join her--and did.

More than once did Elizabeth, in her ramble within the Park, unexpectedly meet Mr. Darcy. She felt all the perverseness of the mischance that should bring him where no one else was brought, and, to prevent its ever happening again, took care to inform him at first that it was a favourite haunt of hers. How it could occur a second time, therefore, was very odd! Yet it did, and even a third. It seemed like wilful ill-nature, or a voluntary penance.... (Ch. 33)