[foldgutter addon] Recompute fold markers on mode change

Closes #6968

Author: marijnh

addon/fold/foldgutter.js

@@ -21,6 +21,7 @@
       cm.off("fold", onFold);
       cm.off("unfold", onFold);
       cm.off("swapDoc", onChange);
+      cm.off("optionChange", optionChange);
     }
     if (val) {
       cm.state.foldGutter = new State(parseOptions(val));
@@ -31,6 +32,7 @@
       cm.on("fold", onFold);
       cm.on("unfold", onFold);
       cm.on("swapDoc", onChange);
+      cm.on("optionChange", optionChange);
     }
   });
 
@@ -120,6 +122,10 @@
     else cm.foldCode(Pos(line, 0), opts);
   }
 
+  function optionChange(cm, option) {
+    if (option == "mode") onChange(cm)
+  }
+
   function onChange(cm) {
     var state = cm.state.foldGutter;
     if (!state) return;