fix: MathML for stretchy accents. #2990 (#2991)

* fix #2990

fix make stretchycode to textNode bug.

* fix: fix stretchy.js@mathMLnode line too long

* chore(screenshotter): update stretchy snapshots

* fix: remove dup '\' in cdrightarrow cdleftarrow and cdlongequal

* test: add stretchy MathML builder test case

Co-authored-by: nferzhuang <nferzhuang@tencent.com>
Co-authored-by: Ron Kok <ronkok@comcast.net>

src/stretchy.js

@@ -54,14 +54,16 @@ const stretchyCodePoint: {[string]: string} = {
     xrightleftarrows: "\u21c4",
     xrightequilibrium: "\u21cc",  // Not a perfect match.
     xleftequilibrium: "\u21cb",   // None better available.
-    "\\\\cdrightarrow": "\u2192",
-    "\\\\cdleftarrow": "\u2190",
-    "\\\\cdlongequal": "=",
+    "\\cdrightarrow": "\u2192",
+    "\\cdleftarrow": "\u2190",
+    "\\cdlongequal": "=",
 };
 
 const mathMLnode = function(label: string): mathMLTree.MathNode {
     const node = new mathMLTree.MathNode(
-        "mo", [new mathMLTree.TextNode(stretchyCodePoint[label])]);
+        "mo",
+        [new mathMLTree.TextNode(stretchyCodePoint[label.replace(/^\\/, '')])],
+    );
     node.setAttribute("stretchy", "true");
     return node;
 };

test/katex-spec.js

@@ -2245,6 +2245,15 @@ describe("A stretchy and non-shifty accent builder", function() {
     });
 });
 
+describe("A stretchy MathML builder", function() {
+    it("should properly render stretchy accents", function() {
+        const tex = `\\widetilde{ABCD}`;
+        const tree = getParsed(tex);
+        const markup = buildMathML(tree, tex, defaultOptions).toMarkup();
+        expect(markup).toContain('<mo stretchy="true">~</mo>');
+    });
+});
+
 describe("An under-accent parser", function() {
     it("should not fail", function() {
         expect("\\underrightarrow{x}").toParse();