IMO that's where alpine.js complements HTMX, you can use HTMX mainly for interfacing with a server and then use alpine.js for any special interaction that does not require a round-trip with the server.
That's my thought as well. Anything that needs new data from the server uses HTMX and if you need to manipulate something already on the page use Alpine, Jquery, vanillajs, or whatever other framework you prefer.
