修复pdf生成的公式横线缺失问题

app/templates/exercise_export/blank_exercise.html.erb

@@ -4,17 +4,33 @@
   <meta charset="utf-8">
   <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
 
-  <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.10.0-rc.1/katex.min.css">
+  <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.11.1/katex.min.css">
   <script src="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.11.1/katex.min.js"></script>
   <script src="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.11.1/contrib/auto-render.min.js"></script>
   <script>
     document.addEventListener("DOMContentLoaded", function() {
       renderMathInElement(document.getElementById('markdown_content'), {
+        displayMode: true,
         delimiters: [
           {left: "$$", right: "$$", display: true},
           {left: "$", right: "$", display: false}
          ]
       });
+      function fix_frac_line(){
+          var win = window;
+          var frac_lines = document.getElementsByClassName("frac-line");
+          for (var i = 0; i < frac_lines.length; i++) {
+              var frac = frac_lines[i];
+              if (win.getComputedStyle) {
+                  style = win.getComputedStyle(frac, '');
+                  if (style['border-bottom-width'] == '0px') {
+                      frac.style['border-bottom-width'] = '1px';
+                  }
+              }
+          }
+      };
+
+      fix_frac_line();
     })
   </script>
 </head>