content: Parse data-animated on image-preview HTML

This might've been done more simply by just adding the boolean value
as a new field like `thumbnailAnimated` on ImagePreviewNode.

But this value works closely with some structured data in thumbnail
URL paths, here in image previews and also in the upcoming "inline
images" feature, #1913. So this new ImageThumbnailLocator class
seemed like a helpful way to present the data to consumers.

Author: chrisbobbe

lib/model/content.dart

@@ -539,7 +539,7 @@ 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 +551,15 @@ 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.
+  /// [ImageThumbnailLocator.urlPath] is a relative URL string.
+  /// 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 +576,61 @@ 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.
+@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 +1433,7 @@ class _ZulipContentParser {
     if (imgElement.className == 'image-loading-placeholder') {
       return ImagePreviewNode(
         srcUrl: href,
-        thumbnailUrl: null,
+        thumbnail: null,
         loading: true,
         originalWidth: null,
         originalHeight: null,
@@ -1411,19 +1445,21 @@ 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 +1483,7 @@ class _ZulipContentParser {
 
     return ImagePreviewNode(
       srcUrl: srcUrl,
-      thumbnailUrl: thumbnailUrl,
+      thumbnail: thumbnail,
       loading: false,
       originalWidth: originalWidth,
       originalHeight: originalHeight,

lib/widgets/content.dart

@@ -637,7 +637,7 @@ class MessageImagePreview extends StatelessWidget {
 
     // TODO image hover animation
     final srcUrl = node.srcUrl;
-    final thumbnailUrl = node.thumbnailUrl;
+    final thumbnailUrl = node.thumbnail?.urlPath;
     final store = PerAccountStoreWidget.of(context);
     final resolvedSrcUrl = store.tryResolveUrl(srcUrl);
     final resolvedThumbnailUrl = thumbnailUrl == null