Posts like this really mean "this doesn't work like I expect it to based on my background with some other technology".
But in this case, I tried in earnest to use nextjs for a project with auth & stripe, etc. this past week, and I can't believe how frustrating it is to get stupid things like modal dialogs to work properly in the client.
I have tons of experience with React SPAs. But the client/server divide in Next remains quite inscrutable to me to the extent that I'm just going to start again with Django (where I nearly started it in the first place).
So yes, it doesn't work like I expect it to either...
Huh? Modals work the same way they do anywhere else.