Just posting this for future me and other travelers.

In JS I was used to trying to detect the "load" event as a trigger to do stuff when the page has loaded but that has already fired by the time you would set up a listener so you can just push the event directly inside the mount function (the JS one).

In app.js

const Hooks = {
  ScreenSize: {
    mounted() {
      // If you want to listen on resize and push the same event
      window.addEventListener("resize", (e) => {
        this.pushEvent("page-size", {
          screenWidth: window.innerWidth,
          screenHeight: window.innerHeight,
        });
      });

      this.pushEvent("page-size", {
        screenWidth: window.innerWidth,
        screenHeight: window.innerHeight,
      });
    },
  },
};
// Amended boilerplate...
let csrfToken = document
  .querySelector("meta[name='csrf-token']")
  .getAttribute("content");
let liveSocket = new LiveSocket("/live", Socket, {
  params: { _csrf_token: csrfToken },
  hooks: Hooks,
});

Somewhere on the page tie in the event to the named hook. This is in a heex file or a render function. It doesn’t have to be a div.

<div phx-hook="ScreenSize" id="screen-size">

Then in your :live_view module put in a function to handle the event.

def handle_event(
        "page-size",
        %{"screenWidth" => screen_width, "screenHeight" => screen_height},
        socket
      ) do
    # Whatever your logic is ....

    {:noreply, socket}
end

As a side note (not part of the solution). if you really need to listen for a ‘done loading’ event, "phx:page-loading-stop" might work better. I can imagine this potentially being useful on the JS side, not so much the Phoenix side.

I hope this helps!