Add \scriptspace after superscripts and subscripts

Summary: Plain TeX assigns `\scriptspace=0.5pt`, which is used in Rules 18bcd in the TeXbook's Appendix G.

Test Plan:
- Look at `x^{x^{x_{x_{x_{x_x}}}}}\biggr|`, see a gap that looks similar to the one produced by real TeX.
- Look at the huxley tests, see that they look reasonable. Make sure that all changes to other huxley tests are now correct, and only changed because of this.

Reviewers: emily

Reviewed By: emily

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D12763

buildTree.js

@@ -196,6 +196,11 @@ var groupTypes = {
             p = fontMetrics.metrics.sup2;
         }
 
+        var multiplier = Style.TEXT.sizeMultiplier *
+                options.style.sizeMultiplier;
+        // \scriptspace is 0.5pt = 0.05em * 10pt/em
+        var scriptspace = 0.05 / multiplier + "em";
+
         var supsub;
 
         if (!group.value.sup) {
@@ -206,6 +211,7 @@ var groupTypes = {
                 sub.height - 0.8 * fontMetrics.metrics.xHeight);
 
             subwrap.style.top = v + "em";
+            subwrap.style.marginRight = scriptspace;
 
             subwrap.depth = subwrap.depth + v;
             subwrap.height = 0;
@@ -221,6 +227,7 @@ var groupTypes = {
                 sup.depth + 0.25 * fontMetrics.metrics.xHeight);
 
             supwrap.style.top = -u + "em";
+            supwrap.style.marginRight = scriptspace;
 
             supwrap.height = supwrap.height + u;
             supwrap.depth = 0;
@@ -252,6 +259,9 @@ var groupTypes = {
             supwrap.style.top = -u + "em";
             subwrap.style.top = v + "em";
 
+            supwrap.style.marginRight = scriptspace;
+            subwrap.style.marginRight = scriptspace;
+
             supwrap.height = supwrap.height + u;
             supwrap.depth = 0;
 

test/huxley/Huxleyfile.json

@@ -117,5 +117,11 @@
         "name": "VerticalSpacing",
         "screenSize": [1024, 768],
         "url": "http://localhost:7936/test/huxley/test.html?pre=potato<br>blah&post=<br>moo&m=x^{\\Huge y}z"
+    },
+
+    {
+        "name": "SupSubHorizSpacing",
+        "screenSize": [1024, 768],
+        "url": "http://localhost:7936/test/huxley/test.html?m=x^{x^{x}}\\Big|x_{x_{x_{x_{x}}}}\\bigg|x^{x^{x_{x_{x_{x_{x}}}}}}\\bigg|"
     }
 ]

test/huxley/SupSubHorizSpacing.hux/record.json

@@ -0,0 +1,5 @@
+[
+  {
+    "action": "screenshot"
+  }
+]