content: Filter negative margin styles if present on a vlist row

This fixes a bug where if negative margin on a vlist row is present
the widget side code would hit an assert. Displaying the error
(red screen) in debug mode, but in release mode the negative padding
would be applied twice, once by `_KatexNegativeMargin` and another by
`Padding` widget.

Author: rajveermalviya

lib/model/katex.dart

@@ -332,10 +332,7 @@ class _KatexParser {
               }
               final pstrutHeight = pstrutStyles.heightEm ?? 0;
 
-              KatexSpanNode innerSpanNode = KatexSpanNode(
-                styles: styles,
-                text: null,
-                nodes: _parseChildSpans(otherSpans));
+              final KatexSpanNode innerSpanNode;
 
               final marginRightEm = styles.marginRightEm;
               final marginLeftEm = styles.marginLeftEm;
@@ -346,11 +343,17 @@ class _KatexParser {
                 innerSpanNode = KatexSpanNode(
                   styles: KatexSpanStyles(),
                   text: null,
-                  nodes: [
-                    KatexNegativeMarginNode(
-                      leftOffsetEm: marginLeftEm,
-                      nodes: [innerSpanNode]),
-                  ]);
+                  nodes: [KatexNegativeMarginNode(
+                    leftOffsetEm: marginLeftEm,
+                    nodes: [KatexSpanNode(
+                      styles: styles.filter(marginLeftEm: false),
+                      text: null,
+                      nodes: _parseChildSpans(otherSpans))])]);
+              } else {
+                innerSpanNode = KatexSpanNode(
+                  styles: styles,
+                  text: null,
+                  nodes: _parseChildSpans(otherSpans));
               }
 
               rows.add(KatexVlistRowNode(

test/model/content_test.dart

@@ -1280,6 +1280,65 @@ class ContentExample {
       ]),
     ]);
 
+  static const mathBlockKatexNegativeMarginsOnVlistRow = 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), // .reset-size6.size3
+                              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 +2435,7 @@ void main() async {
   testParseExample(ContentExample.mathBlockKatexRaisebox);
   testParseExample(ContentExample.mathBlockKatexNegativeMargin);
   testParseExample(ContentExample.mathBlockKatexLogo);
+  testParseExample(ContentExample.mathBlockKatexNegativeMarginsOnVlistRow);
 
   testParseExample(ContentExample.imageSingle);
   testParseExample(ContentExample.imageSingleNoDimensions);

test/widgets/content_test.dart

@@ -630,6 +630,10 @@ void main() {
           ('E', Offset(31.63, 14.52), Size(14.0, 25.0)),
           ('X', Offset(43.06, 9.85), Size(15.42, 25.0)),
         ]),
+        (ContentExample.mathBlockKatexNegativeMarginsOnVlistRow, skip: true, [
+          ('X', Offset(0.00, 7.04), Size(17.03, 25.00)),
+          ('n', Offset(17.03, 15.90), Size(8.63, 17.00)),
+        ]),
       ];
 
       for (final testCase in testCases) {