katex: Handle nulldelimiter class in spans

Fixes: #1677

Author: rajveermalviya

lib/model/katex.dart

@@ -386,6 +386,7 @@ class _KatexParser {
     final spanClasses = element.className != ''
       ? List<String>.unmodifiable(element.className.split(' '))
       : const <String>[];
+    double? widthEm;
     String? fontFamily;
     double? fontSizeEm;
     KatexSpanFontWeight? fontWeight;
@@ -576,7 +577,11 @@ class _KatexParser {
             _ => throw _KatexHtmlParseError(),
           };
 
-        // TODO handle .nulldelimiter and .delimcenter .
+        case 'nulldelimiter':
+          // .nulldelimiter { display: inline-block; width: 0.12em; }
+          widthEm = 0.12;
+
+        // TODO .delimcenter .
 
         case 'op-symbol':
           // .op-symbol { ... }
@@ -621,6 +626,7 @@ class _KatexParser {
 
     final inlineStyles = _parseInlineStyles(element);
     final styles = KatexSpanStyles(
+      widthEm: widthEm,
       fontFamily: fontFamily,
       fontSizeEm: fontSizeEm,
       fontWeight: fontWeight,
@@ -810,6 +816,8 @@ class KatexSpanColor {
 
 @immutable
 class KatexSpanStyles {
+  final double? widthEm;
+
   // TODO(#1674) does height actually appear on generic spans?
   //   In a corpus, the only occurrences that we don't already handle separately
   //   (i.e. occurrences other than on struts, vlists, etc) seem to be within
@@ -834,6 +842,7 @@ class KatexSpanStyles {
   final KatexSpanColor? color;
 
   const KatexSpanStyles({
+    this.widthEm,
     this.heightEm,
     this.topEm,
     this.marginRightEm,
@@ -849,6 +858,7 @@ class KatexSpanStyles {
   @override
   int get hashCode => Object.hash(
     'KatexSpanStyles',
+    widthEm,
     heightEm,
     topEm,
     marginRightEm,
@@ -864,6 +874,7 @@ class KatexSpanStyles {
   @override
   bool operator ==(Object other) {
     return other is KatexSpanStyles &&
+      other.widthEm == widthEm &&
       other.heightEm == heightEm &&
       other.topEm == topEm &&
       other.marginRightEm == marginRightEm &&
@@ -879,6 +890,7 @@ class KatexSpanStyles {
   @override
   String toString() {
     final args = <String>[];
+    if (widthEm != null) args.add('widthEm: $widthEm');
     if (heightEm != null) args.add('heightEm: $heightEm');
     if (topEm != null) args.add('topEm: $topEm');
     if (marginRightEm != null) args.add('marginRightEm: $marginRightEm');
@@ -893,6 +905,7 @@ class KatexSpanStyles {
   }
 
   KatexSpanStyles filter({
+    bool widthEm = true,
     bool heightEm = true,
     bool verticalAlignEm = true,
     bool topEm = true,
@@ -906,6 +919,7 @@ class KatexSpanStyles {
     bool color = true,
   }) {
     return KatexSpanStyles(
+      widthEm: widthEm ? this.widthEm : null,
       heightEm: heightEm ? this.heightEm : null,
       topEm: topEm ? this.topEm : null,
       marginRightEm: marginRightEm ? this.marginRightEm : null,

lib/widgets/katex.dart

@@ -160,6 +160,9 @@ class _KatexSpan extends StatelessWidget {
     }
 
     widget = SizedBox(
+      width: styles.widthEm != null
+        ? styles.widthEm! * em
+        : null,
       height: styles.heightEm != null
         ? styles.heightEm! * em
         : null,

test/model/katex_test.dart

@@ -643,6 +643,38 @@ class KatexExample extends ContentExample {
           text: '∗'),
       ]),
     ]);
+
+  static final nulldelimiter = KatexExample.block(
+    r'\{left,middle,right}. : spans with "nulldelimiter" class',
+    // https://chat.zulip.org/#narrow/channel/7-test-here/topic/Rajesh/near/2205534
+    r'\left. a \middle. b \right.',
+    '<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><mo fence="true" lspace="0.05em" rspace="0.05em">.</mo><mi>b</mi></mrow><annotation encoding="application/x-tex">\\left. a \\middle. b \\right.</annotation></semantics></math></span>'
+      '<span class="katex-html" aria-hidden="true">'
+        '<span class="base">'
+          '<span class="strut" style="height:0.6944em;"></span>'
+            '<span class="minner">'
+              '<span class="mopen nulldelimiter"></span>'
+              '<span class="mord mathnormal">a</span>'
+              '<span class="nulldelimiter"></span>'
+              '<span class="mord mathnormal">b</span>'
+              '<span class="mclose nulldelimiter"></span></span></span></span></span></span></p>', [
+      KatexSpanNode(nodes: [
+        KatexStrutNode(heightEm: 0.6944, verticalAlignEm: null),
+        KatexSpanNode(nodes: [
+          KatexSpanNode(styles: KatexSpanStyles(widthEm: 0.12), nodes: []),
+          KatexSpanNode(
+            styles: KatexSpanStyles(fontFamily: 'KaTeX_Math', fontStyle: KatexSpanFontStyle.italic),
+            text: 'a'),
+          KatexSpanNode(styles: KatexSpanStyles(widthEm: 0.12), nodes: []),
+          KatexSpanNode(
+            styles: KatexSpanStyles(fontFamily: 'KaTeX_Math', fontStyle: KatexSpanFontStyle.italic),
+            text: 'b'),
+          KatexSpanNode(styles: KatexSpanStyles(widthEm: 0.12), nodes: []),
+        ]),
+      ]),
+    ]);
 }
 
 void main() async {
@@ -663,6 +695,7 @@ void main() async {
   testParseExample(KatexExample.textColor);
   testParseExample(KatexExample.customColorMacro);
   testParseExample(KatexExample.phantom);
+  testParseExample(KatexExample.nulldelimiter);
 
   group('parseCssHexColor', () {
     const testCases = [