Fix flex collapsing margins

tests/layout/test_flex.py

@@ -1735,3 +1735,21 @@ def test_flex_grow_in_flex_column():
     assert body.width == div.width == 10
     assert body.margin_width() == 14
     assert body.margin_height() == 9
+
+
+@assert_no_logs
+def test_flex_collapsing_margin():
+    page, = render_pages('''
+      <p style="margin-bottom: 20px; height: 100px">ABC</p>
+      <article style="display: flex; margin-top: 10px">
+        <div>A</div>
+      </article>
+    ''')
+    html, = page.children
+    body, = html.children
+    p, article = body.children
+    div, = article.children
+    assert p.position_y == 0
+    assert p.height == 100
+    assert article.position_y == 110
+    assert div.position_y == 120

weasyprint/layout/block.py

@@ -10,7 +10,7 @@
 from .grid import grid_layout
 from .inline import iter_line_boxes
 from .min_max import handle_min_max_width
-from .percent import resolve_percentages, resolve_position_percentages
+from .percent import percentage, resolve_percentages, resolve_position_percentages
 from .replaced import block_replaced_box_layout
 from .table import table_layout, table_wrapper_width
 
@@ -501,9 +501,14 @@ def _in_flow_layout(context, box, index, child, new_children, page_is_empty,
         no_margin_collapse = box.is_table_wrapper or isinstance(
             child, (boxes.BlockReplacedBox, boxes.FlexBox, boxes.GridBox))
         if no_margin_collapse:
-            collapsed_margin = collapse_margin(adjoining_margins)
-            child.position_y += collapsed_margin
-            position_y += collapsed_margin
+            if child.style['margin_top'] == 'auto':
+                margin_top = 0
+            else:
+                margin_top = percentage(child.style['margin_top'], box.width)
+            adjoining_margins.append(margin_top)
+            offset_y = collapse_margin(adjoining_margins) - margin_top
+            child.position_y += offset_y
+            position_y += offset_y
             adjoining_margins = []
 
     page_is_empty_with_no_children = page_is_empty and not any(