Support for top-level \newline and \\ in inline math (#1298)

* Support for top-level \newline and \\ in inline math

This was a little tricky because `\\` was defined as an endOfExpression.
Instead made `\\` a termination specific to an array environment.
Outside an array environment, buildHTML handles the `cr` object,
resulting in a `.newline` class.  Currently this turns into a
`display: block` (with appropriate vertical spacing) only in inline math,
matching LaTeX.

* Simplify code

* Fix Jest errors

* NewLine screenshot test

* Bug fix: \\ only works at top level of inline

* Add \newline and \cr to test

* Switch test to pmatrix

* Add vertical space test

* Add \\ vs. \newline tests

* Fix flow errors

* Add \cr test

* Add documentation for \\ at top level

* Comment out newRow

* Fix commenting out

test/errors-spec.js

@@ -208,10 +208,6 @@ describe("Parser.expect calls:", function() {
                    "Expected 'EOF', got '\\end' at position 2:" +
                    " x\\̲e̲n̲d̲{matrix}");
         });
-        it("complains about top-level \\\\", function() {
-            expect("1\\\\2").toFailWithParseError(
-                   "Expected 'EOF', got '\\\\' at position 2: 1\\̲\\̲2");
-        });
         it("complains about top-level &", function() {
             expect("1&2").toFailWithParseError(
                    "Expected 'EOF', got '&' at position 2: 1&̲2");
@@ -292,12 +288,12 @@ describe("environments.js:", function() {
     describe("parseArray", function() {
         it("rejects missing \\end", function() {
             expect("\\begin{matrix}1").toFailWithParseError(
-                   "Expected & or \\\\ or \\end at end of input:" +
+                   "Expected & or \\\\ or \\cr or \\end at end of input:" +
                    " \\begin{matrix}1");
         });
         it("rejects incorrectly scoped \\end", function() {
             expect("{\\begin{matrix}1}\\end{matrix}").toFailWithParseError(
-                   "Expected & or \\\\ or \\end at position 17:" +
+                   "Expected & or \\\\ or \\cr or \\end at position 17:" +
                    " …\\begin{matrix}1}̲\\end{matrix}");
         });
     });

test/katex-spec.js

@@ -3126,6 +3126,18 @@ describe("The \\mathchoice function", function() {
     });
 });
 
+describe("Newlines via \\\\ and \\newline", function() {
+    it("should build \\\\ and \\newline the same", () => {
+        expect("hello \\\\ world").toBuildLike("hello \\newline world");
+        expect("hello \\\\[1ex] world").toBuildLike(
+            "hello \\newline[1ex] world");
+    });
+
+    it("should not allow \\cr at top level", () => {
+        expect("hello \\cr world").toNotParse();
+    });
+});
+
 describe("Symbols", function() {
     it("should parse \\text{\\i\\j}", () => {
         expect("\\text{\\i\\j}").toBuild();

test/screenshotter/ss_data.yaml

@@ -207,6 +207,14 @@ NegativeSpace:
 NestedFractions: |
     \dfrac{\frac{a}{b}}{\frac{c}{d}}\dfrac{\dfrac{a}{b}}
     {\dfrac{c}{d}}\frac{\frac{a}{b}}{\frac{c}{d}}
+NewLine: |
+    \frac{a^2+b^2}{c^2} \newline
+    \frac{a^2+b^2}{c^2} \\[1ex]
+    \begin{pmatrix}
+        a & b \\
+        c & d \cr
+    \end{pmatrix} \\
+    a+b+c+{d+\\e}+f+g
 Not: |
     \begin{array}{l}
         \not=\not>\not\geq\not\in\not<\not\leq\not{abc} \\