Fix nested \dfrac (#1825)

* Fix nested \dfrac

* Update screen shots

src/functions/genfrac.js

@@ -17,7 +17,9 @@ const htmlBuilder = (group, options) => {
     // function used
     let style = options.style;
     if (group.size === "display") {
-        style = Style.DISPLAY;
+        // Get display style as a default.
+        // If incoming style is sub/sup, use style.text() to get correct size.
+        style = style.id > 3 ? style.text() : Style.DISPLAY;
     } else if (group.size === "text" &&
         style.size === Style.DISPLAY.size) {
         // We're in a \tfrac but incoming style is displaystyle, so:
@@ -69,7 +71,7 @@ const htmlBuilder = (group, options) => {
     let numShift;
     let clearance;
     let denomShift;
-    if (style.size === Style.DISPLAY.size) {
+    if (style.size === Style.DISPLAY.size || group.size === "display") {
         numShift = options.fontMetrics().num1;
         if (ruleWidth > 0) {
             clearance = 3 * ruleSpacing;

test/screenshotter/ss_data.yaml

@@ -115,9 +115,10 @@ ExtensibleArrows: |
     \end{array}
 FractionTest: |
     \begin{array}{l}
-        \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12\;{1 \atop 2}\; \cfrac{1}{1+\cfrac{1}{x}} \\[2.5em]
-        {a \brace b} \; {a \brack b} \; \genfrac \{ ]{0.8pt}{0}{a}{b}
-        \; {a \above1.0pt b}
+        \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12\;{1 \atop 2} 
+        \; {a \brace b} \; {a \brack b} \\[1em]
+        \genfrac \{ ]{0.8pt}{0}{a}{b} \; {a \above1.0pt b} \; \cfrac{1}{1+\cfrac{1}{x}} 
+         \xrightarrow[\dfrac g h]{\displaystyle\frac g h} \; \xrightarrow [2.\, \dfrac c d] {1.\, \displaystyle\frac c d}
     \end{array}
 Functions: \sin\cos\tan\ln\log
 Gathered: |