Do not drop the character after U+FFFE or U+FFFF in Font.prototype.encodeString

encodeString has the same surrogate-pair guard that encodeToXmlString had
before #21526: `unicode > 0xd7ff && (unicode < 0xe000 || unicode > 0xfffd)`.
That predicate is also true for U+FFFE and U+FFFF, which are single UTF-16
code units, not surrogate pairs. The extra `i++` then steps over the
character that follows them, so it is silently dropped from the
font-encoded output used when saving or printing a PDF.

For example, encoding a string that is U+FFFF followed by "A", with a font
that has a glyph for both, returns an encoded result ending in "A" on this
branch but drops the "A" on master.

Same fix as #21526: the correct test for a real surrogate pair is
`unicode > 0xffff`, since codePointAt only returns a value at or above
0x10000 for an actual pair. This keeps existing behavior for real surrogate
pairs (e.g. emoji) and the U+FFFD boundary, and only stops the character
after U+FFFE/U+FFFF from being dropped.

Added test/unit/fonts_spec.js, since Font.prototype.encodeString had no
direct unit test. It calls the method on a minimal fake `this` (only
toUnicode/cMap are read), since building a full Font requires a complete
properties/font-file setup that this bug doesn't depend on.

src/core/fonts.js

@@ -3710,7 +3710,7 @@ class Font {
 
     for (let i = 0, ii = str.length; i < ii; i++) {
       const unicode = str.codePointAt(i);
-      if (unicode > 0xd7ff && (unicode < 0xe000 || unicode > 0xfffd)) {
+      if (unicode > 0xffff) {
         // unicode is represented by two uint16
         i++;
       }

test/unit/clitests.json

@@ -26,6 +26,7 @@
     "event_utils_spec.js",
     "fetch_stream_spec.js",
     "font_substitutions_spec.js",
+    "fonts_spec.js",
     "image_utils_spec.js",
     "message_handler_spec.js",
     "metadata_spec.js",

test/unit/fonts_spec.js

@@ -0,0 +1,44 @@
+/* Copyright 2026 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { Font } from "../../src/core/fonts.js";
+import { IdentityToUnicodeMap } from "../../src/core/to_unicode_map.js";
+
+describe("Font", () => {
+  describe("encodeString", () => {
+    // `encodeString` only reads `this.toUnicode` and `this.cMap`, so a
+    // full `Font` (which needs a complete properties/font-file setup) isn't
+    // necessary to exercise it in isolation.
+    function encodeString(str, { cMap = null } = {}) {
+      const fakeFont = {
+        toUnicode: new IdentityToUnicodeMap(0, 0x10ffff),
+        cMap,
+      };
+      return Font.prototype.encodeString.call(fakeFont, str);
+    }
+
+    it("should keep the character after U+FFFE or U+FFFF", () => {
+      expect(encodeString("￿A")).toEqual(["\xffA"]);
+      expect(encodeString("￾B")).toEqual(["\xfeB"]);
+    });
+
+    it("should still treat a real surrogate pair as one code point", () => {
+      // U+1F602 ("😂") is genuinely represented by a surrogate pair; the
+      // character after it must still be kept, and the pair itself must
+      // not be split into its two unpaired halves.
+      expect(encodeString("😂C")).toEqual(["\x02C"]);
+    });
+  });
+});

test/unit/jasmine-boot.js

@@ -72,6 +72,7 @@ async function initializePDFJS(callback) {
       "pdfjs-test/unit/event_utils_spec.js",
       "pdfjs-test/unit/fetch_stream_spec.js",
       "pdfjs-test/unit/font_substitutions_spec.js",
+      "pdfjs-test/unit/fonts_spec.js",
       "pdfjs-test/unit/image_utils_spec.js",
       "pdfjs-test/unit/message_handler_spec.js",
       "pdfjs-test/unit/metadata_spec.js",