content: Use RealmStore.serverThumbnailFormats for thumbnails

lib/api/model/initial_snapshot.dart

@@ -117,6 +117,9 @@ class InitialSnapshot {
 
   final int maxFileUploadSizeMib;
 
+  @JsonKey(defaultValue: []) // TODO(server-9) remove default value
+  final List<ThumbnailFormat> serverThumbnailFormats;
+
   final Uri serverEmojiDataUrl;
 
   final String? realmEmptyTopicDisplayName; // TODO(server-10)
@@ -194,6 +197,7 @@ class InitialSnapshot {
     required this.realmPresenceDisabled,
     required this.realmDefaultExternalAccounts,
     required this.maxFileUploadSizeMib,
+    required this.serverThumbnailFormats,
     required this.serverEmojiDataUrl,
     required this.realmEmptyTopicDisplayName,
     required this.realmUsers,
@@ -262,6 +266,32 @@ class RealmDefaultExternalAccount {
   Map<String, dynamic> toJson() => _$RealmDefaultExternalAccountToJson(this);
 }
 
+/// An item in `server_thumbnail_formats`.
+///
+/// For docs, search for "server_thumbnail_formats:"
+/// in <https://zulip.com/api/register-queue>.
+@JsonSerializable(fieldRename: FieldRename.snake)
+class ThumbnailFormat {
+  ThumbnailFormat({
+    required this.name,
+    required this.maxWidth,
+    required this.maxHeight,
+    required this.animated,
+    required this.format,
+  });
+
+  final String name;
+  final int maxWidth;
+  final int maxHeight;
+  final bool animated;
+  final String format;
+
+  factory ThumbnailFormat.fromJson(Map<String, dynamic> json) =>
+    _$ThumbnailFormatFromJson(json);
+
+  Map<String, dynamic> toJson() => _$ThumbnailFormatToJson(this);
+}
+
 /// An item in `recent_private_conversations`.
 ///
 /// For docs, search for "recent_private_conversations:"

lib/model/content.dart

@@ -5,6 +5,7 @@ import 'package:html/parser.dart';
 
 import '../api/model/model.dart';
 import '../api/model/submessage.dart';
+import '../widgets/image.dart';
 import 'code_block.dart';
 import 'katex.dart';
 
@@ -539,7 +540,7 @@ class ImagePreviewNode extends BlockContentNode {
   const ImagePreviewNode({
     super.debugHtmlNode,
     required this.srcUrl,
-    required this.thumbnailUrl,
+    required this.thumbnail,
     required this.loading,
     required this.originalWidth,
     required this.originalHeight,
@@ -551,15 +552,16 @@ class ImagePreviewNode extends BlockContentNode {
   /// authentication credentials to the request.
   final String srcUrl;
 
-  /// The thumbnail URL of the image.
+  /// The thumbnail URL of the image and whether it has an animated version.
   ///
-  /// This may be a relative URL string. It also may not work without adding
-  /// authentication credentials to the request.
+  /// Use [ImageThumbnailLocatorExtension.resolve] to obtain a suitable URL
+  /// for the current UI need.
+  /// It may not work without adding authentication credentials to the request.
   ///
   /// This will be null if the server hasn't yet generated a thumbnail,
   /// or is a version that doesn't offer thumbnails.
   /// It will also be null when [loading] is true.
-  final String? thumbnailUrl;
+  final ImageThumbnailLocator? thumbnail;
 
   /// A flag to indicate whether to show the placeholder.
   ///
@@ -576,27 +578,64 @@ class ImagePreviewNode extends BlockContentNode {
   bool operator ==(Object other) {
     return other is ImagePreviewNode
       && other.srcUrl == srcUrl
-      && other.thumbnailUrl == thumbnailUrl
+      && other.thumbnail == thumbnail
       && other.loading == loading
       && other.originalWidth == originalWidth
       && other.originalHeight == originalHeight;
   }
 
   @override
   int get hashCode => Object.hash('ImagePreviewNode',
-    srcUrl, thumbnailUrl, loading, originalWidth, originalHeight);
+    srcUrl, thumbnail, loading, originalWidth, originalHeight);
 
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(StringProperty('srcUrl', srcUrl));
-    properties.add(StringProperty('thumbnailUrl', thumbnailUrl));
+    properties.add(DiagnosticsProperty<ImageThumbnailLocator>('thumbnail', thumbnail));
     properties.add(FlagProperty('loading', value: loading, ifTrue: "is loading"));
     properties.add(DoubleProperty('originalWidth', originalWidth));
     properties.add(DoubleProperty('originalHeight', originalHeight));
   }
 }
 
+/// Data to locate an image thumbnail,
+/// and whether the image has an animated version.
+///
+/// Use [ImageThumbnailLocatorExtension.resolve] to obtain a suitable URL
+/// for the current UI need.
+@immutable
+class ImageThumbnailLocator extends DiagnosticableTree {
+  ImageThumbnailLocator({
+    required this.urlPath,
+    required this.hasAnimatedVersion,
+  }) : assert(urlPath.startsWith(urlPathPrefix));
+
+  final String urlPath;
+  final bool hasAnimatedVersion;
+
+  static const urlPathPrefix = '/user_uploads/thumbnail/';
+
+  @override
+  bool operator ==(Object other) {
+    if (other is! ImageThumbnailLocator) return false;
+    return urlPath == other.urlPath
+      && hasAnimatedVersion == other.hasAnimatedVersion;
+  }
+
+  @override
+  int get hashCode => Object.hash(urlPath, hasAnimatedVersion);
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(StringProperty('urlPath', urlPath));
+    properties.add(FlagProperty('hasAnimatedVersion', value: hasAnimatedVersion,
+      ifTrue: 'animatable',
+      ifFalse: 'not animatable'));
+  }
+}
+
 class InlineVideoNode extends BlockContentNode {
   const InlineVideoNode({
     super.debugHtmlNode,
@@ -1399,7 +1438,7 @@ class _ZulipContentParser {
     if (imgElement.className == 'image-loading-placeholder') {
       return ImagePreviewNode(
         srcUrl: href,
-        thumbnailUrl: null,
+        thumbnail: null,
         loading: true,
         originalWidth: null,
         originalHeight: null,
@@ -1411,19 +1450,21 @@ class _ZulipContentParser {
     }
 
     final String srcUrl;
-    final String? thumbnailUrl;
-    if (src.startsWith('/user_uploads/thumbnail/')) {
+    final ImageThumbnailLocator? thumbnail;
+    if (src.startsWith(ImageThumbnailLocator.urlPathPrefix)) {
       // For why we recognize this as the thumbnail form, see discussion:
       //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/documenting.20inline.20images/near/2279872
       srcUrl = href;
-      thumbnailUrl = src;
+      thumbnail = ImageThumbnailLocator(
+        urlPath: src,
+        hasAnimatedVersion: imgElement.attributes['data-animated'] == 'true');
     } else {
       // Known cases this handles:
       // - `src` starts with CAMO_URI, a server variable (e.g. on Zulip Cloud
       //   it's "https://uploads.zulipusercontent.net/" in 2025-10).
       // - `src` matches `href`, e.g. from pre-thumbnailing servers.
       srcUrl = src;
-      thumbnailUrl = null;
+      thumbnail = null;
     }
 
     double? originalWidth, originalHeight;
@@ -1447,7 +1488,7 @@ class _ZulipContentParser {
 
     return ImagePreviewNode(
       srcUrl: srcUrl,
-      thumbnailUrl: thumbnailUrl,
+      thumbnail: thumbnail,
       loading: false,
       originalWidth: originalWidth,
       originalHeight: originalHeight,

lib/model/realm.dart

@@ -32,6 +32,14 @@ mixin RealmStore on PerAccountStoreBase, UserGroupStore {
   Duration get serverTypingStartedWaitPeriod => Duration(milliseconds: serverTypingStartedWaitPeriodMilliseconds);
   int get serverTypingStartedWaitPeriodMilliseconds;
 
+  List<ThumbnailFormat> get serverThumbnailFormats;
+  /// A digest of [serverThumbnailFormats]: sorted by max resolution, ascending,
+  /// and filtered to those with `animated: true`.
+  List<ThumbnailFormat> get sortedAnimatedThumbnailFormats;
+  /// A digest of [serverThumbnailFormats]: sorted by max resolution, ascending,
+  /// and filtered to those with `animated: false`.
+  List<ThumbnailFormat> get sortedStillThumbnailFormats;
+
   //|//////////////////////////////////////////////////////////////
   // Realm settings.
 
@@ -166,6 +174,12 @@ mixin ProxyRealmStore on RealmStore {
   @override
   int get serverTypingStartedWaitPeriodMilliseconds => realmStore.serverTypingStartedWaitPeriodMilliseconds;
   @override
+  List<ThumbnailFormat> get serverThumbnailFormats => realmStore.serverThumbnailFormats;
+  @override
+  List<ThumbnailFormat> get sortedAnimatedThumbnailFormats => realmStore.sortedAnimatedThumbnailFormats;
+  @override
+  List<ThumbnailFormat> get sortedStillThumbnailFormats => realmStore.sortedStillThumbnailFormats;
+  @override
   bool get realmAllowMessageEditing => realmStore.realmAllowMessageEditing;
   @override
   GroupSettingValue? get realmCanDeleteAnyMessageGroup => realmStore.realmCanDeleteAnyMessageGroup;
@@ -230,6 +244,11 @@ class RealmStoreImpl extends HasUserGroupStore with RealmStore {
     serverTypingStartedExpiryPeriodMilliseconds = initialSnapshot.serverTypingStartedExpiryPeriodMilliseconds,
     serverTypingStoppedWaitPeriodMilliseconds = initialSnapshot.serverTypingStoppedWaitPeriodMilliseconds,
     serverTypingStartedWaitPeriodMilliseconds = initialSnapshot.serverTypingStartedWaitPeriodMilliseconds,
+    serverThumbnailFormats = initialSnapshot.serverThumbnailFormats,
+    _sortedAnimatedThumbnailFormats = _filterAndSortThumbnailFormats(
+      initialSnapshot.serverThumbnailFormats, animated: true),
+    _sortedStillThumbnailFormats = _filterAndSortThumbnailFormats(
+      initialSnapshot.serverThumbnailFormats, animated: false),
     realmAllowMessageEditing = initialSnapshot.realmAllowMessageEditing,
     realmCanDeleteAnyMessageGroup = initialSnapshot.realmCanDeleteAnyMessageGroup,
     realmCanDeleteOwnMessageGroup = initialSnapshot.realmCanDeleteOwnMessageGroup,
@@ -374,6 +393,15 @@ class RealmStoreImpl extends HasUserGroupStore with RealmStore {
   @override
   final int serverTypingStartedWaitPeriodMilliseconds;
 
+  @override
+  final List<ThumbnailFormat> serverThumbnailFormats;
+  @override
+  List<ThumbnailFormat> get sortedAnimatedThumbnailFormats => _sortedAnimatedThumbnailFormats;
+  final List<ThumbnailFormat> _sortedAnimatedThumbnailFormats;
+  @override
+  List<ThumbnailFormat> get sortedStillThumbnailFormats => _sortedStillThumbnailFormats;
+  final List<ThumbnailFormat> _sortedStillThumbnailFormats;
+
   @override
   final bool realmAllowMessageEditing;
   @override
@@ -432,6 +460,26 @@ class RealmStoreImpl extends HasUserGroupStore with RealmStore {
     return displayFields.followedBy(nonDisplayFields).toList();
   }
 
+  static List<ThumbnailFormat> _filterAndSortThumbnailFormats(
+    List<ThumbnailFormat> initialServerThumbnailFormats, {
+    required bool animated,
+  }) {
+    return initialServerThumbnailFormats
+      .where((format) => format.animated == animated)
+      .toList()
+      ..sort(_compareThumbnailFormats);
+  }
+
+  /// A comparator to sort formats by max resolution, ascending.
+  ///
+  /// "Max resolution" means
+  /// [ThumbnailFormat.maxWidth] * [ThumbnailFormat.maxHeight].
+  static int _compareThumbnailFormats(ThumbnailFormat a, ThumbnailFormat b) {
+    final aMaxResolution = a.maxWidth * a.maxHeight;
+    final bMaxResolution = b.maxWidth * b.maxHeight;
+    return aMaxResolution - bMaxResolution;
+  }
+
   void handleCustomProfileFieldsEvent(CustomProfileFieldsEvent event) {
     customProfileFields = _sortCustomProfileFields(event.fields);
   }

lib/widgets/content.dart

@@ -637,11 +637,12 @@ class MessageImagePreview extends StatelessWidget {
 
     // TODO image hover animation
     final srcUrl = node.srcUrl;
-    final thumbnailUrl = node.thumbnailUrl;
+    final thumbnailLocator = node.thumbnail;
     final store = PerAccountStoreWidget.of(context);
     final resolvedSrcUrl = store.tryResolveUrl(srcUrl);
-    final resolvedThumbnailUrl = thumbnailUrl == null
-      ? null : store.tryResolveUrl(thumbnailUrl);
+    final resolvedThumbnailUrl = thumbnailLocator?.resolve(context,
+      width: 150, height: 100,
+      animationMode: ImageAnimationMode.animateConditionally);
 
     // TODO if src fails to parse, show an explicit "broken image"