Don't clamp BlueScale up when a font genuinely has small zones

The lower BlueScale clamp from #21343 guarded foundry fonts via
`blueScale < DEFAULT_BLUE_SCALE`, but that lets a near-default value
(e.g. 0.037) with small zones get raised to `0.5 / maxZoneHeight`. On
macOS' Core Text rasterizer this collapses the overshooting glyphs, so
most text disappears (not reproducible on Linux/Windows).

src/core/cff_parser.js

@@ -894,8 +894,17 @@ class CFFParser {
         }
       }
       if (maxZoneHeight > 0) {
+        // The lower bound of AFDKO's valid window is `0.5 / maxZoneHeight`.
+        // When that bound is itself above the default BlueScale the font simply
+        // has small zones (e.g. Eurostile LT Std, or the SofiaPro fonts shipped
+        // with a near-default 0.037): even the default 0.039625 would be
+        // flagged as out-of-range, so this is the rendered intent and forcing
+        // BlueScale up only misaligns/collapses overshooting glyphs (notably
+        // with macOS's Core Text rasterizer). Only apply the lower clamp when
+        // its target does not exceed the default.
+        const lowerBound = 0.5 / maxZoneHeight;
         const minBlueScale =
-          blueScale < DEFAULT_BLUE_SCALE ? 0.5 / maxZoneHeight : -Infinity;
+          lowerBound <= DEFAULT_BLUE_SCALE ? lowerBound : -Infinity;
         const maxBlueScale = 1 / maxZoneHeight;
         const clamped = MathClamp(blueScale, minBlueScale, maxBlueScale);
         if (clamped !== blueScale) {

test/pdfs/.gitignore

@@ -937,3 +937,4 @@
 !checkbox_no_appearance.pdf
 !opt_demo.pdf
 !bug1873345.pdf
+!cff_bluescale_small_zones.pdf

test/unit/cff_parser_spec.js

@@ -22,6 +22,9 @@ import {
   CFFStrings,
   CFFTopDict,
 } from "../../src/core/cff_parser.js";
+import { DefaultFileReaderFactory, TEST_PDFS_PATH } from "./test_utils.js";
+import { PDFDocument } from "../../src/core/document.js";
+import { Ref } from "../../src/core/primitives.js";
 import { SEAC_ANALYSIS_ENABLED } from "../../src/core/fonts_utils.js";
 import { Stream } from "../../src/core/stream.js";
 
@@ -357,6 +360,35 @@ describe("CFFParser", function () {
     );
   });
 
+  it("preserves the BlueScale of an embedded CID font with small zones", async function () {
+    // The embedded CID-keyed CFF pairs a near-default BlueScale of 0.037 with
+    // 12-unit zones; clamping it up to the lower bound breaks rendering on
+    // macOS only, so it's guarded here rather than with a reference image.
+    const data = await DefaultFileReaderFactory.fetch({
+      path: TEST_PDFS_PATH + "cff_bluescale_small_zones.pdf",
+    });
+    const pdfManager = {
+      evaluatorOptions: { isOffscreenCanvasSupported: false },
+      password: null,
+    };
+    const pdfDocument = new PDFDocument(pdfManager, new Stream(data));
+    pdfDocument.parseStartXRef();
+    pdfDocument.xref.parse();
+
+    // Object 8 is the `/FontFile3` (`/CIDFontType0C`) stream in the fixture.
+    const fontProgram = pdfDocument.xref.fetch(Ref.get(8, 0)).getBytes();
+    const embeddedCff = new CFFParser(
+      new Stream(fontProgram),
+      {},
+      SEAC_ANALYSIS_ENABLED
+    ).parse();
+
+    expect(embeddedCff.isCIDFont).toEqual(true);
+    expect(embeddedCff.fdArray[0].privateDict.getByName("BlueScale")).toEqual(
+      0.037
+    );
+  });
+
   it("refuses to add topDict key with invalid value (bug 1068432)", function () {
     const topDict = cff.topDict;
     const defaultValue = topDict.getByName("UnderlinePosition");