content: Handle clusters of images in parseImplicitParagraphBlockContentList

Fixes: #193

Author: sirpengi

lib/model/content.dart

@@ -997,6 +997,7 @@ class _ZulipContentParser {
     assert(_debugParserContext == _ParserContext.block);
     final List<BlockContentNode> result = [];
     final List<dom.Node> currentParagraph = [];
+    List<ImageNode> imageNodes = [];
     void consumeParagraph() {
       final parsed = parseBlockInline(currentParagraph);
       result.add(ParagraphNode(
@@ -1014,9 +1015,19 @@ class _ZulipContentParser {
         continue;
       }
       if (currentParagraph.isNotEmpty) consumeParagraph();
-      result.add(parseBlockContent(node));
+      final block = parseBlockContent(node);
+      if (block is ImageNode) {
+        imageNodes.add(block);
+        continue;
+      }
+      if (imageNodes.isNotEmpty) {
+        result.add(ImageNodeList(imageNodes));
+        imageNodes = [];
+      }
+      result.add(block);
     }
     if (currentParagraph.isNotEmpty) consumeParagraph();
+    if (imageNodes.isNotEmpty) result.add(ImageNodeList(imageNodes));
 
     return result;
   }

lib/widgets/content.dart

@@ -85,6 +85,10 @@ class BlockContentList extends StatelessWidget {
         } else if (node is ImageNodeList) {
           return MessageImageList(node: node);
         } else if (node is ImageNode) {
+          assert(false,
+            "[ImageNode] not allowed in [BlockContentList]. "
+            "It should be wrapped in [ImageNodeList]."
+          );
           return MessageImage(node: node);
         } else if (node is UnimplementedBlockContentNode) {
           return Text.rich(_errorUnimplemented(node));