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

@@ -1321,11 +1321,7 @@ class ContentExample {
                     node: KatexSpanNode(styles: KatexSpanStyles(), text: null, nodes: [
                       KatexNegativeMarginNode(leftOffsetEm: -0.0785, nodes: [
                         KatexSpanNode(
-                          styles: KatexSpanStyles(
-                            marginRightEm: 0.05,
-                            // TODO parser should not emit this `marginLeftEm` here because
-                            //   it already generates `KatexNegativeMarginNode` for handling it.
-                            marginLeftEm: -0.0785),
+                          styles: KatexSpanStyles(marginRightEm: 0.05),
                           text: null, nodes: [
                             KatexSpanNode(
                               styles: KatexSpanStyles(fontSizeEm: 0.7), // .reset-size6.size3

test/widgets/content_test.dart

@@ -630,10 +630,7 @@ void main() {
           ('E', Offset(31.63, 14.52), Size(14.0, 25.0)),
           ('X', Offset(43.06, 9.85), Size(15.42, 25.0)),
         ]),
-        // TODO re-enable this test when parser fixes a bug where
-        //    it emits negative margin in styles, allowing widget
-        //    code to hit an assert.
-        (ContentExample.mathBlockKatexNegativeMarginsOnVlistRow, skip: true, [
+        (ContentExample.mathBlockKatexNegativeMarginsOnVlistRow, skip: false, [
           ('X', Offset(0.00, 7.04), Size(17.03, 25.00)),
           ('n', Offset(17.03, 15.90), Size(8.63, 17.00)),
         ]),