Fix mobile overlay inert ownership (#1352)

frontend/mobile-insights.js

@@ -8,12 +8,25 @@
   let active = false;
   let previous = '#/my-work';
   let retries = 0;
+  let backgroundInert = null;
 
   function attr(element, name, value) {
     if (value === null) element.removeAttribute(name);
     else element.setAttribute(name, value);
   }
 
+  function containBackground() {
+    if (backgroundInert) return;
+    backgroundInert = new Map(o.backgrounds.map(element => [element, element.inert]));
+    backgroundInert.forEach((_wasInert, element) => { element.inert = true; });
+  }
+
+  function releaseBackground() {
+    if (!backgroundInert) return;
+    backgroundInert.forEach((wasInert, element) => { element.inert = wasInert; });
+    backgroundInert = null;
+  }
+
   function render(open, focus = false) {
     const wasActive = active;
     active = Boolean(open && o.mediaQuery.matches);
@@ -28,7 +41,8 @@
     attr(o.root, 'aria-modal', active ? 'true' : null);
     attr(o.root, 'aria-hidden', active || !o.mediaQuery.matches ? null : 'true');
     o.root.inert = !active && o.mediaQuery.matches;
-    o.backgrounds.forEach(element => { element.inert = active; });
+    if (active) containBackground();
+    else releaseBackground();
     o.dock.hidden = active;
     attr(o.hud, 'data-overlay-hidden', active ? 'true' : null);
     if (active) o.closeButton.focus();