Also add u+fff9-u+fffb as special characters

Browser don't display anything for them either

Issue #5900

Author: marijnh

doc/manual.html

@@ -284,7 +284,7 @@ <h2>Configuration</h2>
       should be replaced by a
       special <a href="#option_specialCharPlaceholder">placeholder</a>.
       Mostly useful for non-printing special characters. The default
-      is <code>/[\u0000-\u001f\u007f-\u009f\u00ad\u061c\u200b-\u200f\u2028\u2029\ufeff\ufffc]/</code>.</dd>
+      is <code>/[\u0000-\u001f\u007f-\u009f\u00ad\u061c\u200b-\u200f\u2028\u2029\ufeff\ufff9-\ufffc]/</code>.</dd>
       <dt id="option_specialCharPlaceholder"><code><strong>specialCharPlaceholder</strong>: function(char) → Element</code></dt>
       <dd>A function that, given a special character identified by
       the <a href="#option_specialChars"><code>specialChars</code></a>

src/edit/options.js

@@ -68,7 +68,7 @@ export function defineOptions(CodeMirror) {
     for (let i = newBreaks.length - 1; i >= 0; i--)
       replaceRange(cm.doc, val, newBreaks[i], Pos(newBreaks[i].line, newBreaks[i].ch + val.length))
   })
-  option("specialChars", /[\u0000-\u001f\u007f-\u009f\u00ad\u061c\u200b-\u200f\u2028\u2029\ufeff\ufffc]/g, (cm, val, old) => {
+  option("specialChars", /[\u0000-\u001f\u007f-\u009f\u00ad\u061c\u200b-\u200f\u2028\u2029\ufeff\ufff9-\ufffc]/g, (cm, val, old) => {
     cm.state.specialChars = new RegExp(val.source + (val.test("\t") ? "" : "|\t"), "g")
     if (old != Init) cm.refresh()
   })