Add menu item for fast preview, and check for it in the preview code. Make sure CHTML-preview.js is loaded in MathJax.js if the menu item is selected.

unpacked/MathJax.js

@@ -2466,6 +2466,8 @@ MathJax.Hub.Startup = {
           }
           jax.unshift(name);
         }
+        if (config.menuSettings.CHTMLpreview && !MathJax.Extension["CHTML-preview"])
+          {MathJax.Hub.config.extensions.push("CHTML-preview.js")}
       },MathJax.Hub.config],
       ["Post",this.signal,"End Cookie"]
     );

unpacked/extensions/CHTML-preview.js

@@ -27,6 +27,8 @@
  */
 
 (function (HUB,HTML) {
+  
+  var SETTINGS = HUB.config.menuSettings;
 
   var CHTMLpreview = MathJax.Extension["CHTML-preview"] = {
     version: "1.0",
@@ -38,8 +40,9 @@
     config: HUB.CombineConfig("CHTML-preview",{
       Chunks: {EqnChunk: 10000, EqnChunkFactor: 1, EqnChunkDelay: 0},
       color: "inherit",
-      updateTime: 10, updateDelay: 50,
+      updateTime: 10, updateDelay: 10,
-      messageStyle: "none"
+      messageStyle: "none",
+      disabled: false
     }),
 
     //
@@ -51,20 +54,22 @@
         SVG: this.config.Chunks,
       });
       MathJax.Ajax.Styles({".MathJax_Preview":{color:this.config.color}});
-      var update, delay, style, done;
+      var update, delay, style, done, saved;
       var config = this.config;
       HUB.Register.MessageHook("Begin Math Output",function () {
-        if (!done) {
+        if (!done && !config.disabled && SETTINGS.CHTMLpreview &&
+            SETTINGS.renderer !== "CommonHTML") {
           update = HUB.processUpdateTime; delay = HUB.processUpdateDelay;
           style = HUB.config.messageStyle;
           HUB.processUpdateTime = config.updateTime;
           HUB.processUpdateDelay = config.updateDelay;
           HUB.Config({messageStyle: config.messageStyle});
           MathJax.Message.Clear(0,0);
+          saved = true;
         }
       });
       HUB.Register.MessageHook("End Math Output",function () {
-        if (!done) {
+        if (!done && saved) {
           HUB.processUpdateTime = update;
           HUB.processUpdateDelay = delay;
           HUB.Config({messageStyle: style});
@@ -78,6 +83,8 @@
     //  and call the CommonHTML output jax to create the preview
     //
     Preview: function (data) {
+      if (this.config.disabled || !SETTINGS.CHTMLpreview ||
+          SETTINGS.renderer === "CommonHTML") return;
       var preview = data.script.previousSibling;
       if (!preview || preview.className !== MathJax.Hub.config.preRemoveClass) {
         preview = HTML.Element("span",{className:MathJax.Hub.config.preRemoveClass});
@@ -85,7 +92,6 @@
       }
       preview.innerHTML = "";
       return this.postFilter(preview,data);
-      return data;
     },
     postFilter: function (preview,data) {
       try {

unpacked/extensions/MathMenu.js

@@ -964,7 +964,7 @@
       "The MathJax contextual menu will be disabled, but you can " +
       "Alt-Click on an expression to obtain the MathJax menu instead."]
   };
-
+  
   /*************************************************************/
   /*************************************************************/
 
@@ -1092,14 +1092,16 @@
           ITEM.RADIO("HTML-CSS",  "renderer", {action: MENU.Renderer}),
           ITEM.RADIO("Fast HTML", "renderer", {action: MENU.Renderer, value:"CommonHTML"}),
           ITEM.RADIO("MathML",    "renderer", {action: MENU.Renderer, value:"NativeMML"}),
-          ITEM.RADIO("SVG",       "renderer", {action: MENU.Renderer})
+          ITEM.RADIO("SVG",       "renderer", {action: MENU.Renderer}),
+          ITEM.RULE(),
+          ITEM.CHECKBOX("Fast Preview", "CHTMLpreview")
         ),
         ITEM.SUBMENU("MathPlayer",  {hidden:!HUB.Browser.isMSIE || !CONFIG.showMathPlayer,
                                                     disabled:!HUB.Browser.hasMathPlayer},
           ITEM.LABEL(["MPHandles","Let MathPlayer Handle:"]),
           ITEM.CHECKBOX(["MenuEvents","Menu Events"],             "mpContext", {action: MENU.MPEvents, hidden:!isIE9}),
           ITEM.CHECKBOX(["MouseEvents","Mouse Events"],           "mpMouse",   {action: MENU.MPEvents, hidden:!isIE9}),
-          ITEM.CHECKBOX(["MenuAndMouse","Mouse and Menu Events"], "mpMouse", {action: MENU.MPEvents, hidden:isIE9})
+          ITEM.CHECKBOX(["MenuAndMouse","Mouse and Menu Events"], "mpMouse",   {action: MENU.MPEvents, hidden:isIE9})
         ),
         ITEM.SUBMENU(["FontPrefs","Font Preference"],       {hidden:!CONFIG.showFontMenu},
           ITEM.LABEL(["ForHTMLCSS","For HTML-CSS:"]),