fix(engine): correct PageBackgroundFill y-coordinate for partial-height bands (#75)

A page-background fill with heightRatio < 1.0 was painted from the page
bottom upward instead of from the documented yRatio top edge, so a band
with yRatio=0 rendered at the bottom of the page. PlacedFragment.y is the
PDF-native bottom-left origin (y grows up — see
PdfShapeFragmentRenderHandler), so the top-down ratios now convert via
y = (1 - yRatio - heightRatio) * pageHeight. Full-page and full-height
column fills resolve to y=0, unchanged, so existing fills and visual
baselines are unaffected.

Adds top/bottom/mid-band regression tests — partial-height placement was
previously untested because every factory helper uses heightRatio=1,
where top and bottom are indistinguishable.

Author: DemchaAV

CHANGELOG.md

@@ -12,6 +12,17 @@ follow semantic versioning; release dates are ISO 8601.
   regression baselines, and reusable `Subheadline` /
   `SectionHeader.flatSpacedCaps` widget support.
 
+### Bug fixes
+
+- **`PageBackgroundFill` y-coordinate.** A partial-height page-background
+  fill (`heightRatio < 1.0`) was painted from the page **bottom** upward
+  instead of from the `yRatio` top edge the API documents, so a band with
+  `yRatio = 0` rendered at the bottom of the page. Fills now convert the
+  top-down ratios to the PDF bottom-up origin correctly
+  (`y = (1 - yRatio - heightRatio) * pageHeight`); full-page and
+  full-height column fills are unchanged. Adds top-/bottom-/mid-band
+  regression tests.
+
 ## v1.6.4 — 2026-05-22
 
 Bug fix + structured-block patch. Adds two new public Block types —

src/main/java/com/demcha/compose/document/api/DocumentPageBackgrounds.java

@@ -45,12 +45,24 @@ static LayoutGraph apply(LayoutGraph base, List<PageBackgroundFill> fills) {
         for (int page = 0; page < base.totalPages(); page++) {
             for (int i = 0; i < fills.size(); i++) {
                 PageBackgroundFill fill = fills.get(i);
+                // Fill ratios are top-down (yRatio 0.0 = top edge) but
+                // PlacedFragment.y is the PDF-native bottom-left origin
+                // (y grows up — see PdfShapeFragmentRenderHandler, which
+                // calls addRect(x, y, w, h) with (x, y) as the bottom-left).
+                // A band occupies [yRatio, yRatio + heightRatio] measured
+                // from the top, so its bottom edge measured from the page
+                // bottom is (1 - yRatio - heightRatio) * pageHeight. For a
+                // full-height fill (heightRatio 1.0) this is 0.0 — identical
+                // to the previous behaviour, so existing full-page/column
+                // fills are unaffected.
+                double fragmentY =
+                        (1.0 - fill.yRatio() - fill.heightRatio()) * pageHeight;
                 combined.add(new PlacedFragment(
                         "@page-background[" + page + "][" + i + "]",
                         0,
                         page,
                         fill.xRatio() * pageWidth,
-                        fill.yRatio() * pageHeight,
+                        fragmentY,
                         fill.widthRatio() * pageWidth,
                         fill.heightRatio() * pageHeight,
                         com.demcha.compose.engine.components.style.Margin.zero(),

src/main/java/com/demcha/compose/document/api/PageBackgroundFill.java

@@ -29,9 +29,13 @@
  * narrow accent column over a full-page tint.</p>
  *
  * @param xRatio      0.0 = left edge, 1.0 = right edge
- * @param yRatio      0.0 = top edge, 1.0 = bottom edge
+ * @param yRatio      top edge of the fill: 0.0 = page top, 1.0 = page
+ *                    bottom. The fill extends downward from here by
+ *                    {@code heightRatio}.
  * @param widthRatio  width as a fraction of the canvas width (0..1]
- * @param heightRatio height as a fraction of the canvas height (0..1]
+ * @param heightRatio height as a fraction of the canvas height (0..1].
+ *                    Keep {@code yRatio + heightRatio <= 1.0} so the fill
+ *                    stays within the page.
  * @param color       fill color (required)
  */
 public record PageBackgroundFill(double xRatio,

src/test/java/com/demcha/compose/document/api/PageBackgroundTest.java

@@ -256,6 +256,91 @@ void pageBackgroundFillFactoryHelpersComputeRectsCorrectly() {
                 .isEqualTo(new PageBackgroundFill(0.25, 0.0, 0.5, 1.0, c));
     }
 
+    // -- Partial-height band placement (y-coordinate regression) ---------
+    // yRatio is top-down (0.0 = page top) but PlacedFragment.y is PDF
+    // bottom-up, so a partial band must convert via
+    // (1 - yRatio - heightRatio) * pageHeight. Pre-fix these collapsed to
+    // y == 0 because every factory used heightRatio == 1; these assert real
+    // partial bands land at the correct vertical position.
+
+    @Test
+    void topBandAppearsAtTopOfPage() {
+        DocumentColor band = DocumentColor.of(Color.DARK_GRAY);
+        try (DocumentSession session = GraphCompose.document()
+                .pageSize(400, 300)
+                .margin(DocumentInsets.zero())
+                .pageBackgrounds(List.of(
+                        new PageBackgroundFill(0.0, 0.0, 1.0, 0.16, band)))
+                .create()) {
+
+            session.add(new SpacerNode("Block", 200, 80,
+                    DocumentInsets.zero(), DocumentInsets.zero()));
+            List<PlacedFragment> bg = session.layoutGraph().fragments().stream()
+                    .filter(this::isPageBackgroundFragment)
+                    .toList();
+
+            assertThat(bg).hasSize(1);
+            // yRatio 0 = page top → bottom-left at (1 - 0 - 0.16) * 300 = 252,
+            // NOT 0 (the bottom, which was the pre-fix behaviour).
+            assertThat(bg.get(0).y()).isCloseTo(252.0, within(EPS));
+            assertThat(bg.get(0).height()).isCloseTo(48.0, within(EPS));
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    @Test
+    void bottomBandAppearsAtBottomOfPage() {
+        DocumentColor band = DocumentColor.of(Color.DARK_GRAY);
+        try (DocumentSession session = GraphCompose.document()
+                .pageSize(400, 300)
+                .margin(DocumentInsets.zero())
+                .pageBackgrounds(List.of(
+                        new PageBackgroundFill(0.0, 0.84, 1.0, 0.16, band)))
+                .create()) {
+
+            session.add(new SpacerNode("Block", 200, 80,
+                    DocumentInsets.zero(), DocumentInsets.zero()));
+            List<PlacedFragment> bg = session.layoutGraph().fragments().stream()
+                    .filter(this::isPageBackgroundFragment)
+                    .toList();
+
+            assertThat(bg).hasSize(1);
+            // yRatio 0.84 + heightRatio 0.16 = 1.0 → bottom edge flush with
+            // the page bottom: (1 - 0.84 - 0.16) * 300 = 0.
+            assertThat(bg.get(0).y()).isCloseTo(0.0, within(EPS));
+            assertThat(bg.get(0).height()).isCloseTo(48.0, within(EPS));
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    @Test
+    void midPageBandLandsAtCorrectVerticalPosition() {
+        DocumentColor band = DocumentColor.of(Color.DARK_GRAY);
+        try (DocumentSession session = GraphCompose.document()
+                .pageSize(400, 300)
+                .margin(DocumentInsets.zero())
+                .pageBackgrounds(List.of(
+                        new PageBackgroundFill(0.0, 0.4, 1.0, 0.2, band)))
+                .create()) {
+
+            session.add(new SpacerNode("Block", 200, 80,
+                    DocumentInsets.zero(), DocumentInsets.zero()));
+            List<PlacedFragment> bg = session.layoutGraph().fragments().stream()
+                    .filter(this::isPageBackgroundFragment)
+                    .toList();
+
+            assertThat(bg).hasSize(1);
+            // Band spanning 40%..60% from the top → bottom-left at
+            // (1 - 0.4 - 0.2) * 300 = 120, height 0.2 * 300 = 60.
+            assertThat(bg.get(0).y()).isCloseTo(120.0, within(EPS));
+            assertThat(bg.get(0).height()).isCloseTo(60.0, within(EPS));
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
     private boolean isPageBackgroundFragment(PlacedFragment fragment) {
         return fragment.payload() instanceof ShapeFragmentPayload payload
                 && payload.fillColor() != null