For me, the midpoint is indeed when I shift from a reactive phase to a proactive phase, but I still need a scene, a dramatic event, that triggers this change of perspective for the MC (Main Character, or Main Crew if you have several MC).
So in a way it is both, but the midpoint shift can be subtle, it can be more of a change in attitude -- resisting the reactive fear response, for example. Not running from the ghost, but bravely trying to talk to it.
Instead of reactive-proactive, you can think of this as "quick fixes, that don't work, thoughtful fixes". This reflects human nature. Most of us, presented with a problem, will try some quick fix.
But eventually, if our quick fixes keep working for only a few hours before failing, we look for a more permanent solution. Your MC can do something similar.
Let's work backward to find out why: It makes sense for first responses, when the MC doesn't know very much about the problem or threat, to be whatever the MC can think of at the moment.
But it doesn't make sense for this to be their operating method all the way to the climax. Near the climax, readers want the MC to intentionally defeat the villain (or their dilemma, if there is no sentient villain, like in The Martian). The MC should know what they are doing and act with a strategy or expertise or knowledge. We don't want the MC to win because they caught a lucky break (aka deus ex machina). We want them to win on purpose by being better than the villain (or by beating their dilemma).
That shift from amateur->master is what we are talking about. It can be a shift from student->graduate, or incompetence->mastery, apprentice->master, novice->student, even child->adult (in most coming of age stories). It begins at the end of ACT I, about 25% of the way through the story, and the transition ends about the end of Act II, about 75% of the way through the story. (These aren't exact percentages, just a rule of thumb).
But we are working backward, and at some point in this range, we must reach a tipping point. That is usually done in the center (midpoint of Act II). First the instinctive reactive approach has not been solving the problem. But second, and importantly, you need to ensure your MC is learning something about the problem so they are already transitioning from raw amateur to actually knowing something.
At the midpoint scene is where we see this learning come into play, the MC encounters a scene where they do something deliberately and that works. Obviously it can't be the whole solution, but it signals they are now thinking about the problem.
That transition point has to be in there somewhere. Then we can see the MC increasingly doing and planning things deliberately, even if they fail (and some of them should) they should teach the MC something so they work the next time.
The accumulation of these learning experiences, both in Act IIa and more so in Act IIb, are what lead the MC into their final confrontation, the beginning of Act III, where they will risk it all.
Often you will see the midpoint listed as the "darkest hour" for the MC, you can write it that way. the Reactive phase in Act IIa screws things up so bad that they are forced to think and understand the problem better and take some deliberate last-chance that pays off.
But I feel you don't have to write it that way, the important thing is to reflect the human life experience of some sort of maturation, going from a know-nothing kid to an experienced adult. There are several of these in life, from high-school graduate to college graduate, from single adult to married with children adult, from college student to practicing lawyer, etc.
Act II is a metaphor for getting through one of these bumpy maturation processes, the MC going through trials and tribulations to become the hero they need to be to defeat the villain. But even though there are ups and downs, defeats and screw ups, a maturation process implies an underlying ever-increasing level of understanding, each failure or success teaches the MC something new about their problem (or villain, or themselves).