Adjust \underset alignment and spacing (#1290)

* Support Unicode \ll and \lll

Fixes issue #1271

* Adjust \underset alignment and spacing

Fixes issue #1288.

1.  Fixes vertical alignment by keeping the base element on the baseline.
2. Assigns horizontal spacing to \underset in a manner that aligns with LaTeX `\binrel`

* Fix flow error

* Add comment

* Allow for shift of mathops

* Update screenshots

* Test for nested ordgroups

* Set \stackrel spacing class to mrel

* Fix lint error

* Better determination of atom type

* Add comment.

* Add `suppressBaseShift` to ParseNodeTypes

src/ParseNode.js

@@ -68,6 +68,7 @@ type ParseNodeTypes = {
         limits: boolean,
         symbol: boolean,
         alwaysHandleSupSub?: boolean,
+        suppressBaseShift?: boolean,
         body?: string,
         value?: ParseNode<*>[],
     |},

src/functions.js

@@ -70,26 +70,48 @@ defineFunction("mclass", [
 defineFunction("mclass", ["\\stackrel", "\\overset", "\\underset"], {
     numArgs: 2,
 }, function(context, args) {
-    const mathAxisArg = args[1];
+    const baseArg = args[1];
     const shiftedArg = args[0];
 
-    const xAxisOp = new ParseNode("op", {
+    let mclass = "mrel";  // default. May change below.
+    if (context.funcName !== "\\stackrel") {
+        // LaTeX applies \binrel spacing to \overset and \underset.
+        // \binrel spacing varies with (bin|rel|ord) of the atom in the argument.
+        // We'll do the same.
+        let atomType = "";
+        if (baseArg.type === "ordgroup") {
+            atomType = baseArg.value[0].type;
+        } else {
+            atomType = baseArg.type;
+        }
+        if (/^(bin|rel)$/.test(atomType)) {
+            mclass = "m" + atomType;
+        } else {
+            // This may capture some instances in which the baseArg is more than
+            // just a single symbol. Say a \overset inside an \overset.
+            // TODO: A more comprehensive way to determine the baseArg type.
+            mclass = "mord";
+        }
+    }
+
+    const baseOp = new ParseNode("op", {
         type: "op",
         limits: true,
         alwaysHandleSupSub: true,
         symbol: false,
-        value: ordargument(mathAxisArg),
-    }, mathAxisArg.mode);
+        suppressBaseShift: context.funcName !== "\\stackrel",
+        value: ordargument(baseArg),
+    }, baseArg.mode);
 
     const supsub = new ParseNode("supsub", {
-        base: xAxisOp,
+        base: baseOp,
         sup: context.funcName === "\\underset" ? null : shiftedArg,
         sub: context.funcName === "\\underset" ? shiftedArg : null,
     }, shiftedArg.mode);
 
     return {
         type: "mclass",
-        mclass: "mrel",
+        mclass: mclass,
         value: [supsub],
     };
 });

src/functions/op.js

@@ -72,8 +72,9 @@ const htmlBuilder = (group, options) => {
     // If content of op is a single symbol, shift it vertically.
     let baseShift = 0;
     let slant = 0;
-    if (base instanceof domTree.symbolNode) {
-        // Shift the symbol so its center lies on the axis (rule 13). It
+    if (base instanceof domTree.symbolNode && !group.value.suppressBaseShift) {
+        // We suppress the shift of the base of \overset and \underset. Otherwise,
+        // shift the symbol so its center lies on the axis (rule 13). It
         // appears that our fonts have the centers of the symbols already
         // almost on the axis, so these numbers are very small. Note we
         // don't actually apply this here, but instead it is used either in

test/screenshotter/ss_data.yaml

@@ -218,7 +218,7 @@ OpLimits: |
 OverUnderline: x\underline{x}\underline{\underline{x}}\underline{x_{x_{x_x}}}\underline{x^{x^{x^x}}}\overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{\underline{x}}\underline{\overline{x}}}
 OverUnderset: |
     \begin{array}{l}
-    x\overset?=1 \quad \underset{*}{x}^2 \\
+    x\overset?=1 \quad \underset{*}{x}^2 \quad \overset{a}{b}b\underset{a}{b}b \\
     {\displaystyle\lim_{t\underset{>0}\to0}}\\
     a+b+c+d\overset{b+c=0}\longrightarrow a+d\\
     \overset { x = y } { \sqrt { a b } }