Add styled legal dialogs with unchanged supplied content

assets/legal.mjs

@@ -0,0 +1,83 @@
+const dialogs = new Map(
+  [...document.querySelectorAll('.legal-dialog')].map(dialog => [dialog.id, dialog]),
+);
+let returnFocus;
+
+function syncDialog() {
+  const next = dialogs.get(location.hash.slice(1));
+  let closed = false;
+
+  for (const dialog of dialogs.values()) {
+    if (dialog.open && dialog !== next) {
+      dialog.close();
+      closed = true;
+    }
+  }
+
+  if (next && !next.open) {
+    returnFocus ??= document.querySelector(`[data-legal-open][aria-controls="${next.id}"]`);
+    next.showModal();
+    next.querySelector('.legal-copy').scrollTop = 0;
+    next.querySelector('h1').focus({ preventScroll: true });
+  } else if (!next && closed) {
+    returnFocus?.focus({ preventScroll: true });
+    returnFocus = undefined;
+  }
+}
+
+function dismissDialog() {
+  history.replaceState(null, '', location.pathname + location.search);
+  syncDialog();
+}
+
+for (const link of document.querySelectorAll('[data-legal-open]')) {
+  link.addEventListener('click', event => {
+    if (event.button !== 0 || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
+    event.preventDefault();
+    returnFocus = link;
+    history.pushState(null, '', link.hash);
+    syncDialog();
+  });
+}
+
+for (const dialog of dialogs.values()) {
+  dialog.querySelector('[data-legal-close]').addEventListener('click', event => {
+    event.preventDefault();
+    dismissDialog();
+  });
+  dialog.addEventListener('cancel', event => {
+    event.preventDefault();
+    dismissDialog();
+  });
+  dialog.addEventListener('keydown', event => {
+    if (event.key !== 'Tab' || event.altKey || event.ctrlKey || event.metaKey) return;
+    const controls = [...dialog.querySelectorAll('a[href], button:not([disabled]), [tabindex="0"]')];
+    const first = controls[0];
+    const last = controls.at(-1);
+    const active = document.activeElement;
+    if (!controls.includes(active) || (event.shiftKey && active === first)
+      || (!event.shiftKey && active === last)) {
+      event.preventDefault();
+      (event.shiftKey ? last : first)?.focus();
+    }
+  });
+
+  // Close only when both ends of a click are on the backdrop, not a text-selection drag.
+  let startedOutside = false;
+  const outside = event => {
+    const rect = dialog.getBoundingClientRect();
+    return event.clientX < rect.left || event.clientX > rect.right
+      || event.clientY < rect.top || event.clientY > rect.bottom;
+  };
+  dialog.addEventListener('pointerdown', event => {
+    startedOutside = event.target === dialog && outside(event);
+  });
+  dialog.addEventListener('click', event => {
+    if (startedOutside && event.target === dialog && outside(event)) dismissDialog();
+    startedOutside = false;
+  });
+}
+
+window.addEventListener('hashchange', syncDialog);
+window.addEventListener('popstate', syncDialog);
+syncDialog();