Add \jot lineskip to aligned environment, switch contents to displaystyle, and add gathered (#725)

src/buildHTML.js

@@ -592,6 +592,9 @@ groupTypes.array = function(group, options) {
 
     // Vertical spacing
     const baselineskip = 12 * pt; // see size10.clo
+    // Default \jot from ltmath.dtx
+    // TODO(edemaine): allow overriding \jot via \setlength (#687)
+    const jot = 3 * pt;
     // Default \arraystretch from lttab.dtx
     // TODO(gagern): may get redefined once we have user-defined macros
     const arraystretch = utils.deflt(group.value.arraystretch, 1);
@@ -632,6 +635,12 @@ groupTypes.array = function(group, options) {
                 gap = 0;
             }
         }
+        // In AMS multiline environments such as aligned and gathered, rows
+        // correspond to lines that have additional \jot added to the
+        // \baselineskip via \openup.
+        if (group.value.addJot) {
+            depth += jot;
+        }
 
         outrow.height = height;
         outrow.depth = depth;

src/environments.js

@@ -216,13 +216,20 @@ defineEnvironment("aligned", {
     let res = {
         type: "array",
         cols: [],
+        addJot: true,
     };
-    res = parseArray(context.parser, res);
+    res = parseArray(context.parser, res, "display");
+    // Count number of columns = maximum number of cells in each row.
+    // At the same time, prepend empty group {} at beginning of every second
+    // cell in each row (starting with second cell) so that operators become
+    // binary.  This behavior is implemented in amsmath's \start@aligned.
     const emptyGroup = new ParseNode("ordgroup", [], context.mode);
     let numCols = 0;
     res.value.body.forEach(function(row) {
         for (let i = 1; i < row.length; i += 2) {
-            row[i].value.unshift(emptyGroup);
+            // Modify ordgroup node within styling node
+            const ordgroup = row[i].value.value[0];
+            ordgroup.value.unshift(emptyGroup);
         }
         if (numCols < row.length) {
             numCols = row.length;
@@ -245,3 +252,20 @@ defineEnvironment("aligned", {
     }
     return res;
 });
+
+// A gathered environment is like an array environment with one centered
+// column, but where rows are considered lines so get \jot line spacing
+// and contents are set in \displaystyle.
+defineEnvironment("gathered", {
+}, function(context) {
+    let res = {
+        type: "array",
+        cols: [{
+            type: "align",
+            align: "c",
+        }],
+        addJot: true,
+    };
+    res = parseArray(context.parser, res, "display");
+    return res;
+});

test/screenshotter/ss_data.yaml

@@ -73,6 +73,15 @@ DisplayStyle: |
 Exponents: a^{a^a_a}_{a^a_a}
 FractionTest: \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12\;{1 \atop 2}
 Functions: \sin\cos\tan\ln\log
+Gathered: |
+    \begin{gathered}
+      x = \frac{1}{2} \\
+      y = \sum_{i=1}^n i
+    \end{gathered} \approx
+    \begin{array}{c}
+      x = \frac{1}{2} \\
+      y = \sum_{i=1}^n i
+    \end{array}
 GreekLetters: \alpha\beta\gamma\omega
 GroupMacros:
     macros: