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

@@ -313,10 +313,7 @@ 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;
@@ -327,11 +324,17 @@ class _KatexParser {
                 innerSpanNode = KatexSpanNode(
                   styles: KatexSpanStyles(),
                   text: null,
-                  nodes: [
-                    KatexNegativeMarginNode(
-                      leftOffsetEm: marginLeftEm,
-                      nodes: [innerSpanNode]),
-                  ]);
+                  nodes: [KatexNegativeMarginNode(
+                    leftOffsetEm: marginLeftEm,
+                    nodes: [KatexSpanNode(
+                      styles: styles.filter(topEm: false, marginLeftEm: false),
+                      text: null,
+                      nodes: _parseChildSpans(otherSpans))])]);
+              } else {
+                innerSpanNode = KatexSpanNode(
+                  styles: styles.filter(topEm: false),
+                  text: null,
+                  nodes: _parseChildSpans(otherSpans));
               }
 
               rows.add(KatexVlistRowNode(

test/model/content_test.dart

@@ -2575,10 +2575,7 @@ void main() async {
   testParseExample(ContentExample.mathBlockKatexSubSuperScript);
   testParseExample(ContentExample.mathBlockKatexRaisebox);
   testParseExample(ContentExample.mathBlockKatexNegativeMargins);
-  testParseExample(ContentExample.mathBlockNegativeMarginsOnVlistRow,
-    // TODO fix parser to filter negative margins styles from
-    //   vlist row span styles.
-    skip: true);
+  testParseExample(ContentExample.mathBlockNegativeMarginsOnVlistRow);
 
   testParseExample(ContentExample.imageSingle);
   testParseExample(ContentExample.imageSingleNoDimensions);