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tests/concrete/test_canvas_lifecycle.py

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+"""
+Regression tests for the page draw/canvas lifecycle (spec S3).
+
+add_child invalidates the canvas but left _draw pointing at it, and the draw
+property only rebuilt when _draw was None. Callers therefore received a context
+bound to a discarded image while page._canvas stayed None - which is how images
+inside table cells ended up as grey placeholders: table_layouter passed
+canvas=None through to the cell renderer.
+
+Fixing that alone would make layout allocate a full-page canvas per line, since
+layout measures text through the page. Measurement now goes through a dedicated
+scratch context.
+"""
+
+import pytest
+from PIL import Image
+
+from pyWebLayout.abstract.block import Image as AbstractImage, Paragraph
+from pyWebLayout.abstract.inline import Word
+from pyWebLayout.concrete.page import Page
+from pyWebLayout.layout.document_layouter import DocumentLayouter
+from pyWebLayout.style import Font
+from pyWebLayout.style.page_style import PageStyle
+
+
+@pytest.fixture
+def font():
+    return Font(font_size=12)
+
+
+@pytest.fixture
+def page():
+    return Page(size=(400, 600), style=PageStyle())
+
+
+def paragraph_of(font, count=40):
+    paragraph = Paragraph(font)
+    for i in range(count):
+        paragraph.add_word(Word(f"word{i}", font))
+    return paragraph
+
+
+class TestDrawIsNeverStale:
+
+    def test_draw_matches_canvas_after_add_child(self, page, font):
+        page.draw  # force canvas creation
+        DocumentLayouter(page).layout_paragraph(paragraph_of(font))
+
+        assert page.draw.im is page._canvas.im, \
+            "draw must be bound to the page's current canvas"
+
+    def test_canvas_is_present_after_layout(self, page, font):
+        DocumentLayouter(page).layout_paragraph(paragraph_of(font))
+        page.draw
+
+        assert page._canvas is not None
+
+    def test_repeated_draw_access_is_stable(self, page):
+        first = page.draw
+        assert page.draw is first, "draw must not be rebuilt while the canvas stands"
+
+
+class TestMeasurementDoesNotAllocateCanvases:
+
+    def test_layout_allocates_no_page_canvas(self, page, font, monkeypatch):
+        calls = []
+        original = Page._create_canvas
+
+        def counting(self):
+            calls.append(1)
+            return original(self)
+
+        monkeypatch.setattr(Page, "_create_canvas", counting)
+        DocumentLayouter(page).layout_paragraph(paragraph_of(font, 400))
+
+        assert calls == [], \
+            f"layout allocated {len(calls)} full-page canvases; it should allocate none"
+
+    def test_measurement_context_is_tiny_and_matches_canvas_mode(self, page):
+        scratch = page.measurement_draw
+        assert scratch.im.size == (1, 1)
+        assert scratch.mode == Page._CANVAS_MODE
+
+    def test_measurement_context_is_stable(self, page, font):
+        first = page.measurement_draw
+        DocumentLayouter(page).layout_paragraph(paragraph_of(font))
+        assert page.measurement_draw is first, \
+            "the scratch context must survive canvas invalidation"
+
+
+class TestRenderIsRepeatable:
+
+    def test_two_renders_are_identical(self, page, font):
+        DocumentLayouter(page).layout_paragraph(paragraph_of(font))
+
+        first = page.render().copy()
+        second = page.render().copy()
+
+        assert first.tobytes() == second.tobytes()
+
+
+class TestImageInCellGetsARealCanvas:
+    """The concrete symptom: table images degraded to placeholders."""
+
+    @pytest.fixture
+    def image_path(self, tmp_path):
+        path = tmp_path / "swatch.png"
+        Image.new("RGB", (40, 30), (10, 200, 10)).save(path)
+        return str(path)
+
+    def test_table_after_paragraph_receives_a_canvas(self, page, font, image_path):
+        from pyWebLayout.abstract.block import Table, TableCell, TableRow
+        from pyWebLayout.layout.document_layouter import table_layouter
+
+        layouter = DocumentLayouter(page)
+        layouter.layout_paragraph(paragraph_of(font, 10))
+
+        table = Table()
+        row = TableRow()
+        cell = TableCell()
+        cell.add_block(AbstractImage(image_path))
+        row.add_cell(cell)
+        table.add_row(row)
+
+        # The canvas is invalidated by the preceding add_child; the table must
+        # still be handed a real one.
+        assert table_layouter(table, page) or True  # placement may fail on space
+        assert page._canvas is not None, \
+            "table layout must not run against a None canvas"