It's fairly clear that the activities Julia engages in are of a similar form of protest to Offred in The Handmaid's Tale. It is a more "feminine" form of resistance, the preservation of spirit, rather than the "masculine" outrage and direct opposition to oppression.

It is also fairly clear, if you think about it not very hard, that Julia could have continued exactly as she chose, had she not been drawn into counter-establishment thinking by Winston. Why do I say this? Why am I so confident that Julia's behaviour in and of itself was not a problem?

Because the thought police were well aware of what she was doing. We know for a fact that they were able to use that room many times before they were formally arrested. We know for a fact none of Julia's other lovers were ever arrested (she states they killed themselves out of fear.) She knew exactly how to skirt the rules in a way that did not actually threaten the order, and hence went ignored. They could have been picked up after the first meeting if their liaison was actually a material problem, because the establishment knew precisely what they were up to.

We also know that Julia is more or less a passive participant in the dangerous thought Winston engages in. She conveniently falls asleep whenever he reads to her, and she never engages in conversation with him over it. She is passively buffeted along towards her fate by Winston. It's not so much him overstepping a boundary as her never consenting, but never setting the boundary, in the first place. I am certain that after their meeting with O'Brien, had she had more agency she could have disengaged with the revolutionary politick and stepped away. Her thorough disinterest is not Orwell portraying her as stupid, but as a survivalist. Winston is frustrated by her, but she is right.

Her not having agency is part of Orwell's awareness of the status of women in his time. She is very similar to Julia Comstock, as Julia C passively accepts that her brother takes precedence and she allows him to cadge off her. She embodies the self abjugation that defines the peri and post Victorian feminine experience. Julia 1984 carries over some of this, as (despite being sexually rebellious and liberated) she is not shown to make many decisions on the ideas she is exposed to (despite being shrewd and understanding the danger she is in.) In fact, her remaining with Winston after their meeting with O'Brien is slightly out of character and odd. She understands her place in her society far better than Winston does.

Julia is a mostly unwilling sounding board for Winston's politick. He needs to share his revolutionary sentiment (as he kind of begins to do so before they meet up with O'Brien.) She doesn't say no, but she is also very notable in her refusal to say yes. She is tainted by the presence of these ideas, and that is why the Thought Police pick her up. The existence of dangerous thought that threatens the state is what they seek to stamp out, as having heard it (whether she absorbed it or not) she is a container for that thought, and it must be made certain she will abandon it.

I will be honest, when I wrote this bit of my original comment I had mentally run out of steam, but it is a theory I have spent many, many hours piecing together out of my numerous re-reads of Orwell's work.

I would also appreciate that, in future, if you feel the need to query an idea, you don't dismiss it out of hand as "Nonsense." It's rather rude and non-conducive to good conversations. Your questions are excellent, and I am happy to develop and consider my ideas in light of them, but the out of hand insult is quite negative.