Lightbox: pin and size to visible viewport when framed

The staff-report iframe is expanded to full content height (no nested
scrollbar), so a modal <dialog> centered in its viewport landed in the
middle of the whole document and sat off-screen unless the parent page
was scrolled there. Cross-origin means the iframe can't read the
parent's scroll position itself.

Tab.tpl now broadcasts the iframe's visible slice (webform-mw-viewport,
rAF-throttled on scroll/resize/load and after each height change), and
AttachmentLightbox pins to that slice and fills its height. Falls back
to a fixed box when no viewport message arrives (older extension), and
standalone mode keeps native centering at a taller 85vh.

Extension bumped to 0.3.1 (template change -> re-copy + cv flush on
prod). PRODUCTION_CUTOVER.md change log updated.

civi-extension/webform-mw/templates/CRM/WebformMw/Page/Tab.tpl

@@ -22,7 +22,41 @@
       // Add a little headroom so the report's own bottom padding isn't clipped.
       var h = Math.max(600, Math.floor(d.height) + 24);
       frame.style.height = h + 'px';
+      // Geometry changed — tell the child where its visible slice now is.
+      scheduleViewport();
     }, false);
+
+    // The iframe is expanded to full content height, so the report has no
+    // internal scroll context. A modal opened inside it (the attachment
+    // lightbox) would center in the full iframe and sit off-screen. Broadcast
+    // the iframe's currently-visible region (in the child's own content
+    // coordinates) so the lightbox can pin and size itself to it. The child
+    // listens for 'webform-mw-viewport'.
+    var ticking = false;
+    var raf = window.requestAnimationFrame || function (cb) { return setTimeout(cb, 16); };
+    function postViewport() {
+      ticking = false;
+      if (!frame.contentWindow) return;
+      var r = frame.getBoundingClientRect();
+      var winH = window.innerHeight || document.documentElement.clientHeight;
+      var visibleTop = Math.max(0, r.top);
+      var visibleBottom = Math.min(winH, r.bottom);
+      frame.contentWindow.postMessage({
+        type: 'webform-mw-viewport',
+        // px from the iframe's content top down to the first visible row.
+        top: Math.max(0, -r.top),
+        height: Math.max(0, visibleBottom - visibleTop)
+      }, APP_ORIGIN || '*');
+    }
+    function scheduleViewport() {
+      if (ticking) return;
+      ticking = true;
+      raf(postViewport);
+    }
+    window.addEventListener('scroll', scheduleViewport, { passive: true });
+    window.addEventListener('resize', scheduleViewport);
+    window.addEventListener('load', scheduleViewport);
+    scheduleViewport();
   })();
   </script>
 {else}