Support previewing settings without saving them

pass-settings-to-preview.diff

diff --git a/toolkit/actors/PrintingChild.jsm b/toolkit/actors/PrintingChild.jsm
--- a/toolkit/actors/PrintingChild.jsm
+++ b/toolkit/actors/PrintingChild.jsm
@@ -81,17 +81,18 @@ class PrintingChild extends ActorChild {
   receiveMessage(message) {
     let data = message.data;
     switch (message.name) {
       case "Printing:Preview:Enter": {
         this.enterPrintPreview(
           Services.wm.getOuterWindowWithId(data.windowID),
           data.simplifiedMode,
           data.changingBrowsers,
-          data.lastUsedPrinterName
+          data.lastUsedPrinterName,
+          data.jsonSettings
         );
         break;
       }
 
       case "Printing:Preview:Exit": {
         this.exitPrintPreview();
         break;
       }
@@ -305,22 +306,30 @@ class PrintingChild extends ActorChild {
       }
     });
   }
 
   enterPrintPreview(
     contentWindow,
     simplifiedMode,
     changingBrowsers,
-    lastUsedPrinterName
+    lastUsedPrinterName,
+    jsonSettings
   ) {
     const { docShell } = this;
     try {
       let printSettings = this.getPrintSettings(lastUsedPrinterName);
 
+      let customSettings = JSON.parse(jsonSettings);
+      for (let [name, value] of Object.entries(customSettings)) {
+        try {
+          printSettings[name] = value;
+        } catch (e) {}
+      }
+
       // If we happen to be on simplified mode, we need to set docURL in order
       // to generate header/footer content correctly, since simplified tab has
       // "about:blank" as its URI.
       if (printSettings && simplifiedMode) {
         printSettings.docURL = contentWindow.document.baseURI;
       }
 
       // The print preview docshell will be in a different TabGroup, so
diff --git a/toolkit/components/printing/content/print.js b/toolkit/components/printing/content/print.js
--- a/toolkit/components/printing/content/print.js
+++ b/toolkit/components/printing/content/print.js
@@ -111,17 +111,17 @@ var PrintEventHandler = {
     }
 
     if (isChanged) {
       let PSSVC = Cc["@mozilla.org/gfx/printsettings-service;1"].getService(
         Ci.nsIPrintSettingsService
       );
 
       if (flags) {
-        PSSVC.savePrintSettingsToPrefs(this.settings, true, flags);
+        // PSSVC.savePrintSettingsToPrefs(this.settings, true, flags);
         this.updatePrintPreview();
       }
 
       document.dispatchEvent(
         new CustomEvent("print-settings", {
           detail: this.viewSettings,
         })
       );
@@ -136,21 +136,36 @@ var PrintEventHandler = {
         );
       }
       // else there's already an update queued.
     } else {
       this._previewUpdatingPromise = this._updatePrintPreview();
     }
   },
 
+  getJSONSettings() {
+    let obj = {};
+    for (let [prop, value] of Object.entries(this.settings)) {
+      if (
+        this.settings.hasOwnProperty(prop) &&
+        typeof value != "function" &&
+        !prop.startsWith("k") // TODO: Support settings that start with k.
+      ) {
+        obj[prop] = value;
+      }
+    }
+    return JSON.stringify(obj);
+  },
+
   async _updatePrintPreview() {
     let numPages = await PrintUtils.updatePrintPreview(
       this.sourceBrowser,
       this.previewBrowser,
-      this.settings
+      this.settings,
+      this.getJSONSettings()
     );
     document.dispatchEvent(
       new CustomEvent("page-count", { detail: { numPages } })
     );
 
     if (this._queuedPreviewUpdatePromise) {
       // Now that we're done, the queued update (if there is one) will start.
       this._previewUpdatingPromise = this._queuedPreviewUpdatePromise;
diff --git a/toolkit/components/printing/content/printUtils.js b/toolkit/components/printing/content/printUtils.js
--- a/toolkit/components/printing/content/printUtils.js
+++ b/toolkit/components/printing/content/printUtils.js
@@ -190,17 +190,17 @@ var PrintUtils = {
    * @param printPreviewBrowser
    *        The Browser that contains the print preview.
    * @param printSettings
    *        The nsIPrintSettings object to be used to render the preview.
    *        Note: Only printerName is currently used.
    *
    * @return {Promise<Integer>} The number of pages that were rendered in the preview.
    */
-  updatePrintPreview(sourceBrowser, printPreviewBrowser, printSettings) {
+  updatePrintPreview(sourceBrowser, printPreviewBrowser, printSettings, jsonSettings) {
     return new Promise(resolve => {
       printPreviewBrowser.messageManager.addMessageListener(
         "Printing:Preview:UpdatePageCount",
         function done(message) {
           printPreviewBrowser.messageManager.removeMessageListener(
             "Printing:Preview:UpdatePageCount",
             done
           );
@@ -210,16 +210,17 @@ var PrintUtils = {
 
       printPreviewBrowser.messageManager.sendAsyncMessage(
         "Printing:Preview:Enter",
         {
           changingBrowsers: false,
           lastUsedPrinterName: printSettings.printerName,
           simplifiedMode: false,
           windowID: sourceBrowser.outerWindowID,
+          jsonSettings,
         }
       );
     });
   },
 
   /**
    * Initialize a print, this will open the tab modal UI if it is enabled or
    * defer to the native dialog/silent print.