Fence KaTeX math expressions

Flexmark would parse and corrupt the contents of KaTeX math expressions.
This commit circumvents the problem by fencing the math expressions.

Fixes gsantner#1389 and gsantner#1393.

app/src/main/java/net/gsantner/markor/format/markdown/MarkdownTextConverter.java

@@ -193,7 +193,8 @@ public String convertMarkup(String markup, Context context, boolean isExportInLi
         if (appSettings.isMarkdownMathEnabled() && markup.contains("$")) {
             head += HTML_KATEX_INCLUDE;
             onLoadJs += JS_KATEX;
-            markup = markup.replaceAll("(?ms)^([$]{2}.*?[$]{2})$", "<div>\n$1\n</div>");
+            // Temporarily fence math expressions by masking them as code enclosed in math tags.
+            markup = markup.replaceAll("(?ms)(?<![[:graph:]])(([$]{1,2}).*?\\2)", "<math>`$1`</math>");
         }
 
         // Enable View (block) code syntax highlighting
@@ -239,6 +240,15 @@ public String convertMarkup(String markup, Context context, boolean isExportInLi
             }
         }
 
+        // After render changes: Math
+        if (converted.contains("<math><code>$") || converted.contains("<math>`$")) {
+            converted = converted.replaceAll("(?ms)<math><code>(([$]{1,2}).*?\\2)</code></math>", "$1");
+            // Math tags semm to be fenced when found at the beginning of a line. In this case, the backticks
+            // were not converted into code tags.
+            converted = converted.replaceAll("(?ms)<math>`(([$]{1,2}).*?\\2)`</math>", "$1");
+
+        }
+
         // Deliver result
         return putContentIntoTemplate(context, converted, isExportInLightMode, file, onLoadJs, head);
     }