Add a pref-controlled method to notify embedders that mozPrintCallback has completed (bug 2036265)

Adds a `postMessageAfterPrintCallback` browser option (off by default).
When enabled by Firefox, the print service posts "ready" or "error" to
the window after each page's mozPrintCallback resolves, so embedders
(e.g. print-preview test harnesses) can observe when rendering is done.

Upstreams the viewer-side portion of Phabricator D297837.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Daniel Holbert <dholbert@cs.stanford.edu>

web/app_options.js

@@ -369,6 +369,11 @@ const defaultOptions = {
     value: typeof PDFJSDev === "undefined" || PDFJSDev.test("TESTING"),
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
+  postMessageAfterPrintCallback: {
+    /** @type {boolean} */
+    value: false,
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
   printResolution: {
     /** @type {number} */
     value: 150,

web/firefox_print_service.js

@@ -23,6 +23,7 @@ import {
   BasePrintServiceFactory,
   getXfaHtmlForPrinting,
 } from "./print_utils.js";
+import { AppOptions } from "./app_options.js";
 
 // Creates a placeholder with div and canvas with right size for the page.
 function composePage(
@@ -31,6 +32,7 @@ function composePage(
   size,
   printContainer,
   printResolution,
+  shouldPostMessageAfterPrintCallback,
   optionalContentConfigPromise,
   printAnnotationStoragePromise
 ) {
@@ -94,6 +96,9 @@ function composePage(
             currentRenderTask = null;
           }
           obj.done();
+          if (shouldPostMessageAfterPrintCallback) {
+            window.postMessage("ready", "*");
+          }
         },
         function (reason) {
           if (!(reason instanceof RenderingCancelledException)) {
@@ -112,6 +117,9 @@ function composePage(
           } else {
             obj.done();
           }
+          if (shouldPostMessageAfterPrintCallback) {
+            window.postMessage("error", "*");
+          }
         }
       );
   };
@@ -171,6 +179,9 @@ class FirefoxPrintService {
       return;
     }
 
+    const shouldPostMessageAfterPrintCallback = AppOptions.get(
+      "postMessageAfterPrintCallback"
+    );
     for (let i = 0, ii = pagesOverview.length; i < ii; ++i) {
       composePage(
         pdfDocument,
@@ -178,6 +189,7 @@ class FirefoxPrintService {
         pagesOverview[i],
         printContainer,
         _printResolution,
+        shouldPostMessageAfterPrintCallback,
         _optionalContentConfigPromise,
         _printAnnotationStoragePromise
       );