Make supsubs work correctly with character boxes

Summary:
In rule 18a. of the TeX book, it says we're supposed to set `u` and `v` to 0 if
the nucleus is a character box. Character boxes are loosely defined, but through
experimentation they are anything that contains a single character, including
ordgroups with a single element in them. This change makes KaTeX follow this
rule, and fixes T2404.

Test Plan:
- Make sure the new huxley screenshot looks good
- Make sure none of the huxley tests have changed

Reviewers: alpert

Reviewed By: alpert

Maniphest Tasks: T2404

Differential Revision: http://phabricator.khanacademy.org/D8275

buildTree.js

@@ -75,6 +75,24 @@ var getTypeOfGroup = function(group) {
     }
 };
 
+var isCharacterBox = function(group) {
+    if (group == null) {
+        return false;
+    } else if (group.type === "mathord" ||
+               group.type === "textord" ||
+               group.type === "bin" ||
+               group.type === "rel" ||
+               group.type === "open" ||
+               group.type === "close" ||
+               group.type === "punct") {
+        return true;
+    } else if (group.type === "ordgroup") {
+        return group.value.length === 1 && isCharacterBox(group.value[0]);
+    } else {
+        return false;
+    }
+};
+
 var groupTypes = {
     mathord: function(group, options, prev) {
         return makeSpan(
@@ -144,8 +162,13 @@ var groupTypes = {
             var subwrap = makeSpan(["msub"], [submid]);
         }
 
-        var u = base.height - fontMetrics.metrics.supDrop;
-        var v = base.depth + fontMetrics.metrics.subDrop;
+        if (isCharacterBox(group.value.base)) {
+            var u = 0;
+            var v = 0;
+        } else {
+            var u = base.height - fontMetrics.metrics.supDrop;
+            var v = base.depth + fontMetrics.metrics.subDrop;
+        }
 
         var p;
         if (options.style === Style.DISPLAY) {

test/huxley/Huxleyfile

@@ -42,3 +42,6 @@ url=http://localhost:7936/test/huxley/test.html?m=\KaTeX
 
 [RlapBug]
 url=http://localhost:7936/test/huxley/test.html?m=\frac{\rlap{x}}{2}
+
+[SupSubCharacterBox]
+url=http://localhost:7936/test/huxley/test.html?m=a_2f_2{f}_2{aa}_2{af}_2

test/huxley/SupSubCharacterBox.huxley/record.json

@@ -0,0 +1 @@
+{"py/object": "huxley.run.Test", "screen_size": {"py/tuple": [1024, 768]}, "steps": [{"py/object": "huxley.steps.ScreenshotTestStep", "index": 0, "offset_time": 0}]}