From d1d179bb93ca91934daa0487ef81eee50448ca76 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A2u=20Cao?= <raucao@kosmos.org>
Date: Wed, 22 Apr 2026 12:02:44 +0400
Subject: [PATCH] Lazy-load place photos

Only preload photos in view as well as the next one(s), not all of them
---
 app/components/place-photos-carousel.gjs      |  6 +-
 app/modifiers/fade-in-image.js                | 67 ++++++++++++++++---
 .../components/place-photos-carousel-test.gjs |  2 +-
 3 files changed, 60 insertions(+), 15 deletions(-)

diff --git a/app/components/place-photos-carousel.gjs b/app/components/place-photos-carousel.gjs
index d4ac75e..46e5b6d 100644
--- a/app/components/place-photos-carousel.gjs
+++ b/app/components/place-photos-carousel.gjs
@@ -123,11 +123,11 @@ export default class PlacePhotosCarousel extends Component {
                   {{#if photo.thumbUrl}}
                     <source
                       media="(max-width: 768px)"
-                      srcset={{photo.thumbUrl}}
+                      data-srcset={{photo.thumbUrl}}
                     />
                   {{/if}}
                   <img
-                    src={{photo.url}}
+                    data-src={{photo.url}}
                     class="place-header-photo landscape"
                     alt={{@name}}
                     {{fadeInImage photo.url}}
@@ -136,7 +136,7 @@ export default class PlacePhotosCarousel extends Component {
               {{else}}
                 {{! Portrait uses thumb everywhere if available }}
                 <img
-                  src={{if photo.thumbUrl photo.thumbUrl photo.url}}
+                  data-src={{if photo.thumbUrl photo.thumbUrl photo.url}}
                   class="place-header-photo portrait"
                   alt={{@name}}
                   {{fadeInImage (if photo.thumbUrl photo.thumbUrl photo.url)}}
diff --git a/app/modifiers/fade-in-image.js b/app/modifiers/fade-in-image.js
index 8269c10..119b415 100644
--- a/app/modifiers/fade-in-image.js
+++ b/app/modifiers/fade-in-image.js
@@ -7,24 +7,69 @@ export default modifier((element, [url]) => {
   element.classList.remove('loaded');
   element.classList.remove('loaded-instant');
 
-  // Create an off-DOM image to reliably check cache status
-  // without waiting for the actual DOM element to load it
-  const img = new Image();
-  img.src = url;
-
-  if (img.complete) {
-    // Already in browser cache, skip the animation
-    element.classList.add('loaded-instant');
-    return;
-  }
+  let observer;
 
   const handleLoad = () => {
-    element.classList.add('loaded');
+    // Only apply the fade-in animation if it wasn't already loaded instantly
+    if (!element.classList.contains('loaded-instant')) {
+      element.classList.add('loaded');
+    }
   };
 
   element.addEventListener('load', handleLoad);
 
+  const loadWhenVisible = (entries, obs) => {
+    entries.forEach((entry) => {
+      if (entry.isIntersecting) {
+        // Stop observing once we start loading
+        obs.unobserve(element);
+
+        // Check if the image is already in the browser cache
+        // Create an off-DOM image to reliably check cache status
+        // without waiting for the actual DOM element to load it
+        const img = new Image();
+        img.src = url;
+
+        if (img.complete) {
+          // Already in browser cache, skip the animation
+          element.classList.add('loaded-instant');
+        }
+
+        // If this image is inside a <picture> tag, we also need to swap <source> tags
+        const parent = element.parentElement;
+        if (parent && parent.tagName === 'PICTURE') {
+          const sources = parent.querySelectorAll('source');
+          sources.forEach((source) => {
+            if (source.dataset.srcset) {
+              source.srcset = source.dataset.srcset;
+            }
+          });
+        }
+
+        // Swap data-src to src to trigger the actual network fetch (or render from cache)
+        if (element.dataset.src) {
+          element.src = element.dataset.src;
+        } else {
+          // Fallback if data-src wasn't used but the modifier was called
+          element.src = url;
+        }
+      }
+    });
+  };
+
+  // Setup Intersection Observer to only load when the image enters the viewport
+  observer = new IntersectionObserver(loadWhenVisible, {
+    root: null, // Use the viewport as the root
+    rootMargin: '100px 100%', // Load one full viewport width ahead/behind
+    threshold: 0, // Trigger immediately when any part enters the expanded margin
+  });
+
+  observer.observe(element);
+
   return () => {
     element.removeEventListener('load', handleLoad);
+    if (observer) {
+      observer.disconnect();
+    }
   };
 });
diff --git a/tests/integration/components/place-photos-carousel-test.gjs b/tests/integration/components/place-photos-carousel-test.gjs
index 2c8f5c5..6c3488d 100644
--- a/tests/integration/components/place-photos-carousel-test.gjs
+++ b/tests/integration/components/place-photos-carousel-test.gjs
@@ -43,7 +43,7 @@ module('Integration | Component | place-photos-carousel', function (hooks) {
     assert.dom('.carousel-slide').exists({ count: 1 }, 'it renders one slide');
     assert
       .dom('img.place-header-photo')
-      .hasAttribute('src', 'photo1.jpg', 'it renders the photo');
+      .hasAttribute('data-src', 'photo1.jpg', 'it sets the data-src correctly');
 
     // There should be no chevrons when there's only 1 photo
     assert