I went through the Squarespace loop as a returner (two year career gap for family reasons) and was honestly more nervous about the behavioral round than the technical ones. Posting what I found because the behavioral prep resources for Squarespace are thin.
There was one dedicated behavioral round, 45 minutes. The interviewer was from the engineering org, not HR. So the questions had a technical flavor even when they were behavioral.
Questions I remember or can closely paraphrase: Tell me about a time you had to push back on a product decision. What happened and what was the outcome. Describe a project where the technical requirements changed mid-way. How did you adapt? When have you had to learn something unfamiliar quickly to unblock your team? Tell me about a time you disagreed with how your team approached a problem, and what you did.
Theme across all of them: they want to see that you have opinions, can articulate them, and can handle friction without blowing things up. Squarespace seems to care a lot about low-drama collaboration. I got the sense they've had experiences with high-ego engineers who created problems and they are screening for the opposite.
They also asked one question that felt like a values alignment probe: "what does good engineering culture mean to you." No wrong answer exactly, but I'd think about it before you go in.
For the gap question: I addressed it early, briefly, and they didn't press. They asked one follow-up about whether I'd kept up with anything technical, I mentioned a side project, and that was it. I didn't feel penalized for the gap at all, which honestly surprised me.
I did not get an offer in this particular loop (I think I stumbled on the conflict question, I was too diplomatic and never said what I actually thought), but the behavioral round specifically felt fair.