Don't crash when small columns are wrapped in absolute blocks

Fix #897.

weasyprint/layout/columns.py

@@ -158,6 +158,11 @@ def columns_layout(context, box, max_position_y, skip_stack, containing_block,
                     context, column_box, box.content_box_y() + height,
                     column_skip_stack, containing_block, device_size,
                     page_is_empty, [], [], [])
+                if new_box is None:
+                    # We didn't render anything. Give up and use the max
+                    # content height.
+                    height *= count
+                    continue
                 column_skip_stack = resume_at
 
                 # Get the empty space at the bottom of the column box

weasyprint/tests/test_layout/test_column.py

@@ -360,3 +360,23 @@ def test_columns_regression_3():
     assert div1.children[0].children[0].text == 'B1'
     assert div2.children[0].children[0].text == 'B2'
     assert div3.children[0].children[0].text == 'B3'
+
+
+@assert_no_logs
+def test_columns_regression_4():
+    # Regression test #3 for https://github.com/Kozea/WeasyPrint/issues/897
+    page, = render_pages('''
+      <div style="position:absolute">
+        <div style="column-count:2">
+          <div>a</div>
+        </div>
+      </div>
+    ''')
+    html, = page.children
+    body, = html.children
+    div, = body.children
+    assert div.position_y == 0
+    column1, = div.children
+    assert column1.position_y == 0
+    div1, = column1.children
+    assert div1.position_y == 0