Address CodeRabbit review: a layered Duotone writes its real merged image, for #4935

A Duotone PSD opens flattened from its merged image (its layer records are
not read back), but with Maximize Compatibility off a layered Duotone was
saved with the white placeholder, so reopening it showed white. Its merged
image is now always written, like a document without layer records.

Test: a_layered_duotone_keeps_its_image_without_maximize_compatibility.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/io/src/psd_export.rs

@@ -1679,8 +1679,9 @@ fn document_to_psd_nested(doc: &Document, opts: &PsdExportOptions, depth: u32, c
 
     // Without Maximize Compatibility the merged image is a white placeholder and the composite
     // isn't rendered at all. A document without layer records keeps the real image: its pixels
-    // live nowhere else.
-    let real_merged = opts.maximize_compatibility || ex.records.is_empty();
+    // live nowhere else. So does a Duotone: it opens flattened from that image (its layer records
+    // are not read back).
+    let real_merged = opts.maximize_compatibility || ex.records.is_empty() || duotone_inks.is_some();
     let (mut planes, has_alpha) = if real_merged {
         // Merged composite, rendered and encoded in bands (no full-size float composite). Matting
         // against white only changes pixels with alpha < 1; if some are slightly translucent but

crates/io/tests/psd_structure.rs

@@ -245,6 +245,31 @@ fn a_16_bit_duotone_is_written_as_grayscale() {
     assert!(out.warnings.iter().any(|w| w == "Duotone document written as Grayscale"), "{:?}", out.warnings);
 }
 
+#[test]
+fn a_layered_duotone_keeps_its_image_without_maximize_compatibility() {
+    // A Duotone opens flattened from its merged image (its layer records are not read), so that
+    // image is written in full even when Maximize Compatibility is off, never the placeholder.
+    let mut d = duotone_doc(photocraft_color::SampleType::U8);
+    let mut top = photocraft_doc::Layer::new("Dark", LayerContent::Raster(photocraft_raster::Surface::new(d.pixel_format())));
+    if let LayerContent::Raster(s) = &mut top.content {
+        s.fill_rect(photocraft_geom::Rect::new(0, 0, 2, 2), &photocraft_raster::from_rgba(&d.pixel_format(), [0.0, 0.0, 0.0, 1.0]));
+    }
+    d.layers.push(top);
+    let opts = ExportOptions { psd_maximize_compatibility: false, ..Default::default() };
+    let f = PsdFile::from_bytes(&export(&d, "x.psd", &opts).unwrap().bytes).unwrap();
+    assert_eq!(f.header.color_mode, ColorMode::Duotone);
+    assert!(!f.layers().is_empty(), "the layers are still written");
+    // The grey plate: the left half black (the top layer), the right half the 50% grey background.
+    let plate: Vec<u8> = f.decode_merged().unwrap();
+    assert_eq!(plate.len(), 8);
+    assert!(plate[..2].iter().chain(&plate[4..6]).all(|&v| v == 0), "{plate:?}");
+    assert!(plate[2..4].iter().chain(&plate[6..]).all(|&v| (127..=128).contains(&v)), "{plate:?}");
+    let (back, _) = psd_to_document(&f);
+    let maxed = PsdFile::from_bytes(&export(&d, "x.psd", &ExportOptions::default()).unwrap().bytes).unwrap();
+    assert_eq!(f.decode_merged().unwrap(), maxed.decode_merged().unwrap());
+    assert_eq!(back.duotone.as_ref().map(|x| x.inks.len()), Some(2));
+}
+
 #[test]
 fn a_colour_book_ink_shows_its_lab_equivalent_or_the_grey_plate() {
     let out = export(&duotone_doc(photocraft_color::SampleType::U8), "x.psd", &ExportOptions::default()).unwrap();