Not asking if you even want to see the show before buying tickets would annoy me, like she's assuming you'll just say yes.
Feels like a power play IMO, maybe she is just being ice but would have been nicer to make the decision of what to see together.
I had to deal with this kind of stuff with my OH's mum around that time in our relationship, there were a few power plays n I realised I needed to put boundaries up as she was overstepping a few that I definitely was not comfortable with ie introducing me as her DIL, signing my cards with my first name but his surname, just being a little too overbearing which others would have interpreted as nice but I felt was an overstep, like rather than feeling like it was nice, just felt like she wasnt respecting my wishes.
Me n OH have been together for coming up to 8yrs n are expecting our first kid next year, both of us decided marriage certificates etc aren't important to us and neither of us are planning on leaving but felt she was forcing a relationship between me and her that wasn't sincere.
We spend time together 1-1 sometimes but mostly we do things as a family with my OH n dad as well so it's us 4 rather than just me and her. The times we do things just 1-1 its kinda obv (to me) that we need my OH and his dad to balance the situation cause I'm quite introverted and cant quite handle all her energy by myself.
I'd say go to the theatre but say next time it'd be nice to pick the show together or whatever else you would like to be different about the experience but go for a positive sandwich when saying it tho ie positive, bad news, positive:
'I really enjoyed that show, it would be nice if we picked the show together next time but I had a really nice time tonight, thanks'.