I need to reiterate that this is the moment they tell us Mike is starting to have conflicting feelings for Will that are not platonic. It's this moment. Even if he can't place what the feelings are exactly, this is what is clearly happening.
First off, why tf would you do a shot like this if this is just his platonic buddy? Will walks out of frame, therefore the scene is not about him. As he walks off, the lens focuses clearly to a singular shot of just Mike watching in medium close up. This means his perspective in this moment is going to be important. That this scene may even be just about him in particular.
So, this shot of Will and the girl dancing, is from Mike's perspective. Him watching them.
Because they then show us another perspective in this scene for a moment to let you know who's is who's. Dustin's.
The camera moves from Mike & Dustin frowning and watching to zooming in and panning over to Lucas and Max dancing. And as we know, Dustin originally had a crush on Max, so he is clearly upset by seeing this unfold and watches longingly.
The camera even pans back and zooms specifically in on Dustin in this moment to tell us that he is watching Max & Lucas, and that was his perspective in that moment when the camera was on them dancing.
And then it literally cuts right back onto them zoomed in for good measure, telling us that this moment is Dustin's perspective:
Because then it slowly moves the camera across the room over to Stacy, as if the camera is literally his neck turning, noticing her. Which we know right after this shot, he goes over there to ask her to dance.
Notice how Dustin and Mike's attention are in two different places now. Dustin moves on quickly.
These are two different screenshots of the same moment. Like please look at Mike's and Dustin's faces here. Dustin changes the coarse and is now nervous but determined to go up to Stacy. He forgets Max, because he knows that is never happening now. But poor Mike Wheeler. His sights are still locked on the same spot. On Will dancing with the girl. Please look at this kid's face 😭 it goes from this:
I'm sorry, but this kid is realizing something. There is something going on in that brain of his and possibly even something is his stomach that he can't place or is scared to name. Mind you, let's remember. This entire scene, Mike says absolutely nothing, literally. Starting from the very beginning, when Will was still standing there, up until he sits looking miserable. Anyway, everything in this scene that Mike expresses is through facial expression and body language. Which you know what scene this reminds me of? Hm, oh yeah:
This one, lol. This scene may be Will's chance to speak from the heart, but we don't get as many shots of Will while he is speaking in this scene compared to shots of Mike simply reacting quietly. This kid has been going through it for several years. When he goes through something with high emotions involved, he goes quiet, clearly.
Leaving it on this devastating note before Jane walks in and his spirits lift. I know people most likely brush off literally everything that happens before Jane enters through those doors because he seems happy once he sees her. But this entire scene beforehand is clearly crucial.
Mike looks absolutely devastated before she arrives, so I think many take it as "Oh no, Jane is not here, therefore Mike is depressed", but his mind this entire time was clearly elsewhere, not on Jane, because he was preoccupied with whatever was going on before.
Her walking into the dance made him forget everything. And people seem to conveniently forget that while Mike is having this cute moment with Jane, Will is still off dancing with that girl. Which is why Mike was originally sitting there alone in the first place. Will dancing with that girl instead of sitting there with Mike gave him the chance to sit there alone, looking shattered. Mike clearly seems to instantly regret doing so as he watches them walk onto the dance floor and watches silently. If only Mike hadn't silently nudged Will to respond yes to her, he may have been sitting there alongside him.
For me, this is the moment that is pivotal for how Mike and Will's relationship unfolds over the course of the following seasons. And it all starts with a choice that Mike technically made, because he nudged Will to do it. What does Mike say in S5? "I don't want anymore regrets." Right, cool. Please free Mike Wheeler from his repressive hell 😪