Prevent duplicate post submissions

static/app.js

@@ -0,0 +1,58 @@
+(() => {
+  const formSelector = "form[data-submit-once]";
+
+  function progressIndicator() {
+    return document.getElementById("submit-progress");
+  }
+
+  function resetForm(form) {
+    form.removeAttribute("aria-busy");
+    delete form.dataset.submitting;
+
+    const button = form.querySelector("[data-submit-button]");
+    if (!button) {
+      return;
+    }
+    button.disabled = false;
+    button.removeAttribute("aria-disabled");
+    if (button.dataset.idleLabel) {
+      button.textContent = button.dataset.idleLabel;
+      delete button.dataset.idleLabel;
+    }
+  }
+
+  document.addEventListener("submit", (event) => {
+    const form = event.target.closest(formSelector);
+    if (!form) {
+      return;
+    }
+    if (form.dataset.submitting === "true") {
+      event.preventDefault();
+      return;
+    }
+
+    form.dataset.submitting = "true";
+    form.setAttribute("aria-busy", "true");
+
+    const button = event.submitter || form.querySelector("[data-submit-button]");
+    if (button) {
+      button.dataset.idleLabel = button.textContent;
+      button.textContent = form.dataset.submittingLabel || "Posting…";
+      button.disabled = true;
+      button.setAttribute("aria-disabled", "true");
+    }
+
+    const progress = progressIndicator();
+    if (progress) {
+      progress.hidden = false;
+    }
+  });
+
+  window.addEventListener("pageshow", () => {
+    document.querySelectorAll(formSelector).forEach(resetForm);
+    const progress = progressIndicator();
+    if (progress) {
+      progress.hidden = true;
+    }
+  });
+})();