test: verify screenshot seam alignment; release tile buffers

docs/superpowers/plans/2026-07-07-wabot-modernization.md

@@ -2298,6 +2298,48 @@ class TestChromiumPath:
             if c.args and str(c.args[0]).startswith("window.scrollTo")
         ]
         assert len(scroll_calls) >= 2
+
+    def test_tall_page_stitches_tiles_bottom_aligned(self, tmp_path):
+        driver = MagicMock(name="driver")
+        driver.capabilities = {"browserName": "chrome"}
+        driver.execute_script.side_effect = lambda script: {
+            "return document.body.parentNode.scrollWidth": 100,
+            "return document.body.parentNode.scrollHeight": 150,  # 3 tiles of 60
+            "return document.documentElement.clientWidth": 100,
+            "return window.innerHeight": 60,
+        }.get(script)  # window.scrollTo(...) -> None
+        driver.get_screenshot_as_png.side_effect = [
+            png_bytes(100, 60, (255, 0, 0)),   # tile @ y=0
+            png_bytes(100, 60, (0, 255, 0)),   # tile @ y=60
+            png_bytes(100, 60, (0, 0, 255)),   # tile @ y=120, clamped to paste_y=90
+        ]
+        out = tmp_path / "shot.png"
+        assert save_full_page(driver, str(out)) is True
+        img = Image.open(out)
+        assert img.size == (100, 150)
+        assert img.getpixel((50, 30)) == (255, 0, 0)    # first tile
+        assert img.getpixel((50, 75)) == (0, 255, 0)     # second tile (rows 60-89)
+        assert img.getpixel((50, 140)) == (0, 0, 255)    # third tile, bottom-aligned
+
+    def test_wide_page_clamps_last_column(self, tmp_path):
+        driver = MagicMock(name="driver")
+        driver.capabilities = {"browserName": "chrome"}
+        driver.execute_script.side_effect = lambda script: {
+            "return document.body.parentNode.scrollWidth": 150,  # 2 columns of 100
+            "return document.body.parentNode.scrollHeight": 60,
+            "return document.documentElement.clientWidth": 100,
+            "return window.innerHeight": 60,
+        }.get(script)
+        driver.get_screenshot_as_png.side_effect = [
+            png_bytes(100, 60, (255, 0, 0)),   # column @ x=0
+            png_bytes(100, 60, (0, 0, 255)),   # column @ x=100, clamped to paste_x=50
+        ]
+        out = tmp_path / "shot.png"
+        assert save_full_page(driver, str(out)) is True
+        img = Image.open(out)
+        assert img.size == (150, 60)
+        assert img.getpixel((25, 30)) == (255, 0, 0)     # left column (cols 0-49)
+        assert img.getpixel((140, 30)) == (0, 0, 255)    # right column, clamped (cols 50-149)
 ```
 
 - [ ] **Step 2: Run tests to verify they fail**
@@ -2371,7 +2413,9 @@ def _stitch_chromium(driver, filename: str) -> bool:
         while x < total_width:
             driver.execute_script(f"window.scrollTo({x}, {y})")
             time.sleep(_SCROLL_SETTLE_SECONDS)
-            tile = Image.open(BytesIO(driver.get_screenshot_as_png()))
+            # Tiles are device pixels; the canvas/scroll math is CSS pixels. Assumes
+            # devicePixelRatio == 1 (true for headless chromium in the test target).
+            with Image.open(BytesIO(driver.get_screenshot_as_png())) as tile:
                 # the last row/column can't scroll a full viewport: paste aligned
                 # to the bottom/right edge instead of duplicating content
                 paste_x = min(x, total_width - viewport_width)
@@ -2387,7 +2431,7 @@ def _stitch_chromium(driver, filename: str) -> bool:
 - [ ] **Step 4: Run tests to verify they pass**
 
 Run: `uv run pytest tests/unit/test_screenshot.py -v`
-Expected: 3 passed. Also run `uv run pytest` — all unit tests still pass.
+Expected: 5 passed. Also run `uv run pytest` — all unit tests still pass.
 
 - [ ] **Step 5: Commit**
 
@@ -2949,7 +2993,7 @@ Run: `uv run pytest tests/unit/test_api.py -v`
 Expected: 11 passed
 
 Run: `uv run pytest`
-Expected: full unit suite passes (126 tests: pacing 7, sessions 14, hosts 19, reattach 6, fields 17, page 36, screenshot 3, browser 13, api 11)
+Expected: full unit suite passes (128 tests: pacing 7, sessions 14, hosts 19, reattach 6, fields 17, page 36, screenshot 5, browser 13, api 11)
 
 - [ ] **Step 5: Commit**
 

src/wabot/screenshot.py

@@ -59,7 +59,9 @@ def _stitch_chromium(driver, filename: str) -> bool:
         while x < total_width:
             driver.execute_script(f"window.scrollTo({x}, {y})")
             time.sleep(_SCROLL_SETTLE_SECONDS)
-            tile = Image.open(BytesIO(driver.get_screenshot_as_png()))
+            # Tiles are device pixels; the canvas/scroll math is CSS pixels. Assumes
+            # devicePixelRatio == 1 (true for headless chromium in the test target).
+            with Image.open(BytesIO(driver.get_screenshot_as_png())) as tile:
                 # the last row/column can't scroll a full viewport: paste aligned
                 # to the bottom/right edge instead of duplicating content
                 paste_x = min(x, total_width - viewport_width)

tests/unit/test_screenshot.py

@@ -66,3 +66,45 @@ class TestChromiumPath:
             if c.args and str(c.args[0]).startswith("window.scrollTo")
         ]
         assert len(scroll_calls) >= 2
+
+    def test_tall_page_stitches_tiles_bottom_aligned(self, tmp_path):
+        driver = MagicMock(name="driver")
+        driver.capabilities = {"browserName": "chrome"}
+        driver.execute_script.side_effect = lambda script: {
+            "return document.body.parentNode.scrollWidth": 100,
+            "return document.body.parentNode.scrollHeight": 150,  # 3 tiles of 60
+            "return document.documentElement.clientWidth": 100,
+            "return window.innerHeight": 60,
+        }.get(script)  # window.scrollTo(...) -> None
+        driver.get_screenshot_as_png.side_effect = [
+            png_bytes(100, 60, (255, 0, 0)),   # tile @ y=0
+            png_bytes(100, 60, (0, 255, 0)),   # tile @ y=60
+            png_bytes(100, 60, (0, 0, 255)),   # tile @ y=120, clamped to paste_y=90
+        ]
+        out = tmp_path / "shot.png"
+        assert save_full_page(driver, str(out)) is True
+        img = Image.open(out)
+        assert img.size == (100, 150)
+        assert img.getpixel((50, 30)) == (255, 0, 0)    # first tile
+        assert img.getpixel((50, 75)) == (0, 255, 0)     # second tile (rows 60-89)
+        assert img.getpixel((50, 140)) == (0, 0, 255)    # third tile, bottom-aligned
+
+    def test_wide_page_clamps_last_column(self, tmp_path):
+        driver = MagicMock(name="driver")
+        driver.capabilities = {"browserName": "chrome"}
+        driver.execute_script.side_effect = lambda script: {
+            "return document.body.parentNode.scrollWidth": 150,  # 2 columns of 100
+            "return document.body.parentNode.scrollHeight": 60,
+            "return document.documentElement.clientWidth": 100,
+            "return window.innerHeight": 60,
+        }.get(script)
+        driver.get_screenshot_as_png.side_effect = [
+            png_bytes(100, 60, (255, 0, 0)),   # column @ x=0
+            png_bytes(100, 60, (0, 0, 255)),   # column @ x=100, clamped to paste_x=50
+        ]
+        out = tmp_path / "shot.png"
+        assert save_full_page(driver, str(out)) is True
+        img = Image.open(out)
+        assert img.size == (150, 60)
+        assert img.getpixel((25, 30)) == (255, 0, 0)     # left column (cols 0-49)
+        assert img.getpixel((140, 30)) == (0, 0, 255)    # right column, clamped (cols 50-149)