Delay clearing notifications until a card is actually viewed.

Send the "read" beacon when the card is made visible.

Also take care to only send the "disconnect" beacon if the card is
visible.

ref: https://fizzy.37signals.com/5986089/collections/2/cards/1301

app/javascript/controllers/beacon_controller.js

@@ -4,11 +4,17 @@ import { Controller } from "@hotwired/stimulus"
 export default class extends Controller {
   static values = { url: String }
 
-  connect() {
-    post(this.urlValue, { responseKind: "turbo-stream" })
+  visibilityChanged() {
+    this.#sendBeacon()
   }
 
   disconnect() {
-    post(this.urlValue, { responseKind: "turbo-stream" })
+    this.#sendBeacon()
+  }
+
+  #sendBeacon() {
+    if (!document.hidden) {
+      post(this.urlValue, { responseKind: "turbo-stream" })
+    }
   }
 }