Avoid auto margins in flex layout

Fix #800.

The way auto margins are resolved for the parent is currently really stupid. We
have to find exactly when these margins must be set.

weasyprint/layout/flex.py

@@ -39,13 +39,18 @@ def flex_layout(context, box, max_position_y, skip_stack, containing_block,
     else:
         axis, cross = 'height', 'width'
 
+    margin_left = 0 if box.margin_left == 'auto' else box.margin_left
+    margin_right = 0 if box.margin_right == 'auto' else box.margin_right
+    margin_top = 0 if box.margin_top == 'auto' else box.margin_top
+    margin_bottom = 0 if box.margin_bottom == 'auto' else box.margin_bottom
+
     if getattr(box, axis) != 'auto':
         available_main_space = getattr(box, axis)
     else:
         if axis == 'width':
             available_main_space = (
                 containing_block.width -
-                box.margin_left - box.margin_right -
+                margin_left - margin_right -
                 box.padding_left - box.padding_right -
                 box.border_left_width - box.border_right_width)
         else:
@@ -58,7 +63,7 @@ def flex_layout(context, box, max_position_y, skip_stack, containing_block,
                     main_space = min(main_space, containing_block.height)
             available_main_space = (
                 main_space -
-                box.margin_top - box.margin_bottom -
+                margin_top - margin_bottom -
                 box.padding_top - box.padding_bottom -
                 box.border_top_width - box.border_bottom_width)
 
@@ -75,24 +80,28 @@ def flex_layout(context, box, max_position_y, skip_stack, containing_block,
                     main_space = min(main_space, containing_block.height)
             available_cross_space = (
                 main_space -
-                box.margin_top - box.margin_bottom -
+                margin_top - margin_bottom -
                 box.padding_top - box.padding_bottom -
                 box.border_top_width - box.border_bottom_width)
         else:
             available_cross_space = (
                 containing_block.width -
-                box.margin_left - box.margin_right -
+                margin_left - margin_right -
                 box.padding_left - box.padding_right -
                 box.border_left_width - box.border_right_width)
 
     # Step 3
     children = box.children
-    resolve_percentages(box, containing_block)
-    if box.margin_top == 'auto':
+    parent_box = box.copy_with_children(children)
+    resolve_percentages(parent_box, containing_block)
+    if parent_box.margin_top == 'auto':
         box.margin_top = 0
-    if box.margin_bottom == 'auto':
+    if parent_box.margin_bottom == 'auto':
         box.margin_bottom = 0
-    parent_box = box.copy_with_children(children)
+    if parent_box.margin_left == 'auto':
+        box.margin_left = 0
+    if parent_box.margin_right == 'auto':
+        box.margin_right = 0
     if isinstance(parent_box, boxes.FlexBox):
         blocks.block_level_width(parent_box, containing_block)
     else:

weasyprint/tests/test_layout/test_flex.py

@@ -353,3 +353,11 @@ def test_flex_item_min_height():
         div_3.height ==
         article.height ==
         50)
+
+
+@assert_no_logs
+def test_flex_auto_margin():
+    # Regression test for https://github.com/Kozea/WeasyPrint/issues/800
+    page, = render_pages('<div style="display: flex; margin: auto">')
+    page, = render_pages(
+        '<div style="display: flex; flex-direction: column; margin: auto">')