PDF export: a CMYK page without blending through is an isolated group (#2156)

crates/pdf/src/export.rs

@@ -241,12 +241,13 @@ impl Writer {
         }
         // Page Isolated Blending / Page Knockout Group: the page content is one group (the PDF
         // writer has no page group attributes; a flat PDF 1.3 file has none). In CMYK, transparency
-        // at the top of a page is put in a non-isolated group of its own, whose blending space
-        // `cmyk_blending` rewrites.
+        // at the top of a page is put in a group of its own, whose blending space `cmyk_blending`
+        // rewrites: non-isolated only when blending on the page reaches the paper, since some
+        // readers (CoreGraphics) drop most of a page in a non-isolated group (#2156).
         let doc = ex.doc;
         let page_group = fits && !self.pdf13 && (doc.page_isolate || doc.page_knockout);
         let cmyk_page_group = fits && self.cmyk && !page_group && ex.shows_transparency();
-        if page_group {
+        if page_group || (cmyk_page_group && !Node::children_blend(&doc.layers, &mut |c| c.blends_through())) {
             s.push_isolated();
         } else if cmyk_page_group {
             let all = constant_mask(&mut s, r, 1.0);

crates/pdf/src/tests_cmykblend.rs

@@ -85,3 +85,21 @@ fn constant_alpha_masks_are_white() {
         assert!(t.contains("/S/Alpha") && t.contains("111rg") && !t.contains("000rg"), "grouped {grouped}: {t}");
     }
 }
+
+#[test]
+fn a_cmyk_page_without_blending_through_is_an_isolated_group() {
+    // #2156: CoreGraphics drops most of a page put in a non-isolated group. Without a blend mode
+    // reaching the paper, isolating the page's group changes nothing it draws.
+    let mut d = Document::new_with_mode(100.0, 100.0, ColorMode::Cmyk);
+    add(&mut d, rect(Rect::new(0.0, 0.0, 100.0, 100.0), Color::cmyk(0.0, 1.0, 0.0, 0.0)));
+    let mut top = rect(Rect::new(20.0, 20.0, 80.0, 80.0), Color::cmyk(1.0, 0.0, 0.0, 0.0));
+    top.opacity = 0.6;
+    add(&mut d, top);
+    let t = text(&d);
+    assert!(t.contains("/Itrue") && t.contains("/CS/DeviceCMYK") && !t.contains("/S/Alpha"), "{t}");
+    // Blending that reaches the paper keeps the page's group non-isolated.
+    for grouped in [true, false] {
+        let t = text(&doc(ColorMode::Cmyk, grouped));
+        assert!(!t.contains("/Itrue") && t.contains("/S/Alpha"), "grouped {grouped}: {t}");
+    }
+}