content: Handle multiple math blocks in <p>

Fixes: #1130

Author: rajveermalviya

lib/model/content.dart

@@ -1451,6 +1451,60 @@ class _ZulipContentParser {
     return tableNode ?? UnimplementedBlockContentNode(htmlNode: tableElement);
   }
 
+  List<BlockContentNode> parseMathBlocks(dom.Element element) {
+    assert(_debugParserContext == _ParserContext.block);
+    assert(element.localName == 'p');
+    assert((() {
+      final first = element.nodes.first;
+      return first is dom.Element
+        && first.localName == 'span'
+        && first.className == 'katex-display';
+    })());
+    final debugHtmlNode = kDebugMode ? element : null;
+
+    final blocks = <BlockContentNode>[];
+
+    // The case with the `<br>\n` can happen when at the end of a quote;
+    // it seems like a glitch in the server's Markdown processing,
+    // so hopefully there just aren't any further such glitches.
+    bool hasTrailingBreakNewline = false;
+    if (element.nodes
+          case [..., dom.Element(localName: 'br'), dom.Text(text: '\n')]
+    ) {
+      hasTrailingBreakNewline = true;
+    }
+
+    final length = hasTrailingBreakNewline
+      ? element.nodes.length - 2
+      : element.nodes.length;
+    for (var i = 0; i < length; i++) {
+      final child = element.nodes[i];
+
+      // If there are multiple <span class="katex-display"> nodes in a <p>
+      // each node is interleaved by '\n\n'. Whitespaces are ignored in HTML
+      // on web but each node has `display: block`, which renders each node
+      // on a newline. So do the same here.
+      if (child case dom.Text(text: '\n\n')) continue;
+
+      if (child is! dom.Element) {
+        blocks.add(UnimplementedBlockContentNode(htmlNode: child));
+        continue;
+      }
+      if (child.className != 'katex-display') {
+        blocks.add(UnimplementedBlockContentNode(htmlNode: child));
+        continue;
+      }
+
+      final texSource = parseMath(child, block: true);
+      if (texSource == null) {
+        blocks.add(UnimplementedBlockContentNode(htmlNode: child));
+        continue;
+      }
+      blocks.add(MathBlockNode(texSource: texSource, debugHtmlNode: debugHtmlNode));
+    }
+    return blocks;
+  }
+
   BlockContentNode parseBlockContent(dom.Node node) {
     assert(_debugParserContext == _ParserContext.block);
     final debugHtmlNode = kDebugMode ? node : null;
@@ -1470,23 +1524,6 @@ class _ZulipContentParser {
     }
 
     if (localName == 'p' && className.isEmpty) {
-      // Oddly, the way a math block gets encoded in Zulip HTML is inside a <p>.
-      if (element.nodes case [dom.Element(localName: 'span') && var child, ...]) {
-        if (child.className == 'katex-display') {
-          if (element.nodes case [_]
-                              || [_, dom.Element(localName: 'br'),
-                                     dom.Text(text: "\n")]) {
-            // This might be too specific; we'll find out when we do #190.
-            // The case with the `<br>\n` can happen when at the end of a quote;
-            // it seems like a glitch in the server's Markdown processing,
-            // so hopefully there just aren't any further such glitches.
-            final texSource = parseMath(child, block: true);
-            if (texSource == null) return UnimplementedBlockContentNode(htmlNode: node);
-            return MathBlockNode(texSource: texSource, debugHtmlNode: debugHtmlNode);
-          }
-        }
-      }
-
       final parsed = parseBlockInline(element.nodes);
       return ParagraphNode(debugHtmlNode: debugHtmlNode,
         links: parsed.links,
@@ -1638,6 +1675,16 @@ class _ZulipContentParser {
         continue;
       }
 
+      // Oddly, the way math blocks get encoded in Zulip HTML is inside a <p>.
+      // And there can be multiple math blocks inside the paragraph node, so
+      // handle it explicitly here.
+      if (node case
+        dom.Element(localName: 'p', nodes: [
+          dom.Element(localName: 'span', className: 'katex-display'), ...])) {
+        result.addAll(parseMathBlocks(node));
+        continue;
+      }
+
       final block = parseBlockContent(node);
       if (block is ImageNode) {
         imageNodes.add(block);

test/model/content_test.dart

@@ -506,6 +506,23 @@ class ContentExample {
       '<span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord mathnormal">λ</span></span></span></span></span></p>',
     [MathBlockNode(texSource: r'\lambda')]);
 
+  static const mathBlocksMultipleInParagraph = ContentExample(
+    'math blocks, multiple in paragraph',
+    '```math\na\n\nb\n```',
+    // https://chat.zulip.org/#narrow/channel/7-test-here/topic/.E2.9C.94.20Rajesh/near/2001490
+    '<p>'
+      '<span class="katex-display"><span class="katex">'
+        '<span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi>a</mi></mrow>'
+          '<annotation encoding="application/x-tex">a</annotation></semantics></math></span>'
+        '<span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.4306em;"></span><span class="mord mathnormal">a</span></span></span></span></span>\n\n'
+      '<span class="katex-display"><span class="katex">'
+        '<span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi>b</mi></mrow>'
+          '<annotation encoding="application/x-tex">b</annotation></semantics></math></span>'
+        '<span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord mathnormal">b</span></span></span></span></span></p>', [
+      MathBlockNode(texSource: 'a'),
+      MathBlockNode(texSource: 'b'),
+    ]);
+
   static const mathBlockInQuote = ContentExample(
     'math block in quote',
     // There's sometimes a quirky extra `<br>\n` at the end of the `<p>` that
@@ -1471,6 +1488,7 @@ void main() {
 
   testParseExample(ContentExample.mathBlock);
   testParseExample(ContentExample.mathBlockInQuote);
+  testParseExample(ContentExample.mathBlocksMultipleInParagraph);
 
   testParseExample(ContentExample.imageSingle);
   testParseExample(ContentExample.imageSingleNoDimensions);