From b163a07e84906c0b86b13b16cd5bf541c1b00530 Mon Sep 17 00:00:00 2001
From: Nils Schneider <nils.schneider@posteo.de>
Date: Sun, 26 Oct 2025 10:36:47 +0100
Subject: [PATCH] lazy-thumbnails

---
 frontend/src/documents/DocumentsTable.jsx | 88 ++++++++++++++++++++---
 1 file changed, 77 insertions(+), 11 deletions(-)

diff --git a/frontend/src/documents/DocumentsTable.jsx b/frontend/src/documents/DocumentsTable.jsx
index 26993b0..954ae4f 100644
--- a/frontend/src/documents/DocumentsTable.jsx
+++ b/frontend/src/documents/DocumentsTable.jsx
@@ -1,4 +1,4 @@
-import React, { useCallback, useEffect, useMemo, useRef } from 'react';
+import React, { useCallback, useEffect, useMemo, useRef, useState } from 'react';
 import { getAssetFromVersion, resolveDocumentAssetUrl, createAssetView } from '../asset_manager';
 import { getTagColorStyle } from '../utils/colors';
 import { DownloadIcon, EditIcon, ViewListIcon, ViewGridIcon, FolderIcon, TrashIcon } from '../ui/icons';
@@ -13,13 +13,62 @@ const getPageCount = (doc) =>
     ? doc.current_version.metadata.page_count
     : null;
 
+// Detects when an element becomes visible within a scroll container.
+const useLazyVisibility = (rootRef, resetKey) => {
+  const targetRef = useRef(null);
+  const [isVisible, setIsVisible] = useState(false);
+
+  useEffect(() => {
+    setIsVisible(false);
+  }, [resetKey]);
+
+  const rootNode = rootRef?.current || null;
+
+  useEffect(() => {
+    if (isVisible) {
+      return undefined;
+    }
+    const element = targetRef.current;
+    if (!element) {
+      return undefined;
+    }
+    if (typeof window === 'undefined' || typeof IntersectionObserver === 'undefined') {
+      setIsVisible(true);
+      return undefined;
+    }
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        entries.forEach((entry) => {
+          if (entry.isIntersecting) {
+            setIsVisible(true);
+            observer.disconnect();
+          }
+        });
+      },
+      {
+        root: rootNode,
+        rootMargin: '200px 0px',
+        threshold: 0.01,
+      },
+    );
+
+    observer.observe(element);
+    return () => observer.disconnect();
+  }, [isVisible, rootNode, resetKey]);
+
+  return { ref: targetRef, isVisible };
+};
+
 const DocumentThumbnailImage = ({
   document,
   ensureAssetUrl,
   getDocumentAsset,
   alt,
   maxSize = LIST_ICON_SIZE,
+  scrollRootRef = null,
 }) => {
+  const { ref: visibilityRef, isVisible } = useLazyVisibility(scrollRootRef, document?.id);
   const resolvedMaxSize = Math.max(1, Math.round(maxSize || 1));
   const thumbnailAsset = useMemo(
     () => getAssetFromVersion(document?.current_version, 'thumbnail'),
@@ -45,14 +94,15 @@ const DocumentThumbnailImage = ({
     () => ({ width: `${dimensions.width}px`, height: `${dimensions.height}px` }),
     [dimensions.height, dimensions.width],
   );
-  const url = useMemo(
-    () =>
-      resolveDocumentAssetUrl(document, 'thumbnail', {
-        ensureAssetUrl,
-        getAsset: getDocumentAsset,
-      }),
-    [document, ensureAssetUrl, getDocumentAsset],
-  );
+  const url = useMemo(() => {
+    if (!isVisible) {
+      return null;
+    }
+    return resolveDocumentAssetUrl(document, 'thumbnail', {
+      ensureAssetUrl,
+      getAsset: getDocumentAsset,
+    });
+  }, [document, ensureAssetUrl, getDocumentAsset, isVisible]);
 
   const pageCount = getPageCount(document);
   const showMultiPageBadge = Number.isFinite(pageCount) && pageCount > 1;
@@ -62,13 +112,15 @@ const DocumentThumbnailImage = ({
   }
 
   return (
-    <div className="document-thumbnail-wrapper">
+    <div className="document-thumbnail-wrapper" ref={visibilityRef}>
       <div className={innerClasses.join(' ')} style={innerStyle}>
         {url ? (
           <img
             src={url}
             alt={alt || ''}
             className="document-thumbnail"
+            loading="lazy"
+            decoding="async"
             draggable={false}
             onDragStart={(event) => event.preventDefault()}
           />
@@ -142,6 +194,18 @@ const DocumentsTable = ({
     [draggingDocumentIds],
   );
   const scrollRef = useRef(null);
+  const [, forceVisibilityTick] = useState(0);
+  const lastScrollNodeRef = useRef(null);
+  const assignScrollRef = useCallback((node) => {
+    if (lastScrollNodeRef.current === node) {
+      return;
+    }
+    lastScrollNodeRef.current = node;
+    scrollRef.current = node;
+    if (node) {
+      forceVisibilityTick((value) => value + 1);
+    }
+  }, []);
   const isGridView = viewMode === 'grid';
   const gridIconSize = DEFAULT_GRID_ICON_SIZE;
   const handleSetViewMode = useCallback(
@@ -329,7 +393,7 @@ const DocumentsTable = ({
       )}
       <div className="column-body">
         <div
-          ref={scrollRef}
+          ref={assignScrollRef}
           className="documents-scroll"
           onFocus={(event) => {
             if (event.target === scrollRef.current) {
@@ -454,6 +518,7 @@ const DocumentsTable = ({
                       getDocumentAsset={getDocumentAsset}
                       alt={`Thumbnail for ${doc.title || doc.original_name}`}
                       maxSize={gridIconSize}
+                      scrollRootRef={scrollRef}
                     />
                     <div className="document-card__meta">
                       <div
@@ -670,6 +735,7 @@ const DocumentsTable = ({
                           ensureAssetUrl={ensureAssetUrl}
                           getDocumentAsset={getDocumentAsset}
                           alt={`Thumbnail for ${doc.title || doc.original_name}`}
+                          scrollRootRef={scrollRef}
                         />
                       </td>
                       <td className="doc-list__name">