content test: Add test for negative margins on a vlist row in KaTeX content

Skipped currently, because the parser incorrectly doesn't filter
the negative margin styles on the vlist row span after generating a
`KatexNegativeMarginNode` to specifically handle negative margins.

Author: rajveermalviya

test/model/content_test.dart

@@ -1280,6 +1280,87 @@ class ContentExample {
       ]),
     ]);
 
+  static const mathBlockNegativeMarginsOnVlistRow = ContentExample(
+    'math block, KaTeX negative margins on a vlist row',
+    '```math\nX_n\n```',
+    '<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><msub><mi>X</mi><mi>n</mi></msub></mrow><annotation encoding="application/x-tex">X_n</annotation></semantics></math></span>'
+        '<span class="katex-html" aria-hidden="true">'
+          '<span class="base">'
+            '<span class="strut" style="height:0.8333em;vertical-align:-0.15em;"></span>'
+            '<span class="mord">'
+              '<span class="mord mathnormal" style="margin-right:0.07847em;">X</span>'
+              '<span class="msupsub">'
+                '<span class="vlist-t vlist-t2">'
+                  '<span class="vlist-r">'
+                    '<span class="vlist" style="height:0.1514em;">'
+                      '<span style="top:-2.55em;margin-left:-0.0785em;margin-right:0.05em;">'
+                        '<span class="pstrut" style="height:2.7em;"></span>'
+                        '<span class="sizing reset-size6 size3 mtight">'
+                          '<span class="mord mathnormal mtight">n</span></span></span></span>'
+                  '<span class="vlist-s">​</span></span>'
+                  '<span class="vlist-r">'
+                    '<span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span></span></span></span></span></p>', [
+      MathBlockNode(
+        texSource: 'X_n',
+        nodes: [
+          KatexSpanNode(
+            styles: KatexSpanStyles(),
+            text: null,
+            nodes: [
+              KatexStrutNode(heightEm: 0.8333, verticalAlignEm: -0.15),
+              KatexSpanNode(
+                styles: KatexSpanStyles(),
+                text: null,
+                nodes: [
+                  KatexSpanNode(
+                    styles: KatexSpanStyles(
+                      marginRightEm: 0.07847,
+                      fontFamily: 'KaTeX_Math',
+                      fontStyle: KatexSpanFontStyle.italic),
+                    text: 'X',
+                    nodes: null),
+                  KatexSpanNode(
+                    styles: KatexSpanStyles(textAlign: KatexSpanTextAlign.left),
+                    text: null,
+                    nodes: [
+                      KatexVlistNode(
+                        rows: [
+                          KatexVlistRowNode(
+                            verticalOffsetEm: -2.55 + 2.7,
+                            node: KatexSpanNode(
+                              styles: KatexSpanStyles(),
+                              text: null,
+                              nodes: [
+                                KatexNegativeMarginNode(
+                                  leftOffsetEm: -0.0785,
+                                  nodes: [
+                                    KatexSpanNode(
+                                      styles: KatexSpanStyles(marginRightEm: 0.05),
+                                      text: null,
+                                      nodes: [
+                                        KatexSpanNode(
+                                          styles: KatexSpanStyles(fontSizeEm: 0.7),
+                                          text: null,
+                                          nodes: [
+                                            KatexSpanNode(
+                                              styles: KatexSpanStyles(
+                                                fontFamily: 'KaTeX_Math',
+                                                fontStyle: KatexSpanFontStyle.italic),
+                                              text: 'n',
+                                              nodes: null),
+                                          ]),
+                                      ]),
+                                  ]),
+                              ])),
+                        ]),
+                    ]),
+                ]),
+            ]),
+        ]),
+    ]);
+
   static const imageSingle = ContentExample(
     'single image',
     // https://chat.zulip.org/#narrow/stream/7-test-here/topic/Thumbnails/near/1900103
@@ -2376,6 +2457,10 @@ void main() async {
   testParseExample(ContentExample.mathBlockKatexRaisebox);
   testParseExample(ContentExample.mathBlockKatexNegativeMargin);
   testParseExample(ContentExample.mathBlockKatexLogo);
+  testParseExample(ContentExample.mathBlockNegativeMarginsOnVlistRow,
+    // TODO fix parser to filter negative margins styles from
+    //   vlist row span styles.
+    skip: true);
 
   testParseExample(ContentExample.imageSingle);
   testParseExample(ContentExample.imageSingleNoDimensions);