Handle margin-break

Some corner-cases are not handled yet, but that's much better than the current
situation.

Fix #115.

weasyprint/css/properties.py

@@ -124,6 +124,7 @@ INITIAL_VALUES = {
     'break_after': 'auto',
     'break_before': 'auto',
     'break_inside': 'auto',
+    'margin_break': 'auto',
     'orphans': 2,
     'widows': 2,
 

weasyprint/css/validation/properties.py

@@ -301,6 +301,13 @@ def break_inside(keyword):
     return keyword in ('auto', 'avoid', 'avoid-page', 'avoid-column')
 
 
+@property()
+@single_keyword
+def margin_break(keyword):
+    """``margin-break`` property validation."""
+    return keyword in ('auto', 'keep', 'discard')
+
+
 @property(unstable=True)
 @single_token
 def page(token):

weasyprint/layout/__init__.py

@@ -204,6 +204,7 @@ class LayoutContext(object):
         self.excluded_shapes = None  # Not initialized yet
         self.string_set = defaultdict(lambda: defaultdict(lambda: list()))
         self.current_page = None
+        self.forced_break = False
 
         # Cache
         self.strut_layouts = {}

weasyprint/layout/blocks.py

@@ -41,6 +41,15 @@ def block_level_layout(context, box, max_position_y, skip_stack,
         if box.margin_bottom == 'auto':
             box.margin_bottom = 0
 
+        if (context.current_page > 1 and page_is_empty):
+            # TODO: we should take care of cases when this box doesn't have
+            # collaping borders with the first child of the page, see
+            # test_margin_break_clearance.
+            if box.style['margin_break'] == 'discard':
+                box.margin_top = 0
+            elif box.style['margin_break'] == 'auto' and context.forced_break:
+                box.margin_top = 0
+
         collapsed_margin = collapse_margin(
             adjoining_margins + [box.margin_top])
         box.clearance = get_clearance(context, box, collapsed_margin)
@@ -428,11 +437,6 @@ def block_container_layout(context, box, max_position_y, skip_stack,
                 page_name = block_level_page_name(last_in_flow_child, child)
                 if page_name or page_break in (
                         'page', 'left', 'right', 'recto', 'verso'):
-                    if page_break == 'page':
-                        page_break = 'any'
-                    elif page_break not in ('left', 'right', 'recto', 'verso'):
-                        assert page_name
-                        page_break = 'any'
                     page_name = child.page_values()[0]
                     next_page = {'break': page_break, 'page': page_name}
                     resume_at = (index, None)

weasyprint/layout/pages.py

@@ -545,6 +545,7 @@ def make_page(context, root_box, page_type, resume_at, page_number,
     # See http://www.w3.org/TR/CSS21/visuren.html#dis-pos-flo
     assert isinstance(root_box, (boxes.BlockBox, boxes.FlexContainerBox))
     context.create_block_formatting_context()
+    context.current_page = page_number
     page_is_empty = True
     adjoining_margins = []
     positioned_boxes = []  # Mixed absolute and fixed
@@ -722,16 +723,20 @@ def remake_page(index, context, root_box, html, cascaded_styles,
     page_state = copy.deepcopy(initial_page_state)
     next_page_name = initial_next_page['page']
     first = index == 0
+    # TODO: handle recto/verso and add test
     blank = ((initial_next_page['break'] == 'left' and right_page) or
              (initial_next_page['break'] == 'right' and not right_page))
     if blank:
         next_page_name = None
     side = 'right' if right_page else 'left'
-    page_type = PageType(
-        side, blank, first, name=(next_page_name or None))
+    page_type = PageType(side, blank, first, name=(next_page_name or None))
     set_page_type_computed_styles(
         page_type, cascaded_styles, computed_styles, html)
 
+    context.forced_break = (
+        initial_next_page['break'] != 'any' or initial_next_page['page'])
+    context.margin_clearance = False
+
     # make_page wants a page_number of index + 1
     page_number = index + 1
     page, resume_at, next_page = make_page(

weasyprint/tests/test_layout/test_page.py

@@ -427,6 +427,75 @@ def test_page_breaks_complex_8():
     assert div_4.height == 20
 
 
+@assert_no_logs
+@pytest.mark.parametrize('break_after, margin_break, margin_top', (
+    ('page', 'auto', 0),
+    ('auto', 'auto', 5),
+    ('page', 'keep', 5),
+    ('auto', 'keep', 5),
+    ('page', 'discard', 0),
+    ('auto', 'discard', 0),
+))
+def test_margin_break(break_after, margin_break, margin_top):
+    page_1, page_2 = render_pages('''
+      <style>
+        @page { size: 70px; margin: 0 }
+        div { height: 63px; margin: 5px 0 8px;
+              break-after: %s; margin-break: %s }
+      </style>
+      <section>
+        <div></div>
+      </section>
+      <section>
+        <div></div>
+      </section>
+    ''' % (break_after, margin_break))
+    html, = page_1.children
+    body, = html.children
+    section, = body.children
+    div, = section.children
+    assert div.margin_top == 5
+
+    html, = page_2.children
+    body, = html.children
+    section, = body.children
+    div, = section.children
+    assert div.margin_top == margin_top
+
+
+@pytest.mark.xfail
+@assert_no_logs
+def test_margin_break_clearance():
+    page_1, page_2 = render_pages('''
+      <style>
+        @page { size: 70px; margin: 0 }
+        div { height: 63px; margin: 5px 0 8px; break-after: page }
+      </style>
+      <section>
+        <div></div>
+      </section>
+      <section>
+        <div style="border-top: 1px solid black">
+          <div></div>
+        </div>
+      </section>
+    ''')
+    html, = page_1.children
+    body, = html.children
+    section, = body.children
+    div, = section.children
+    assert div.margin_top == 5
+
+    html, = page_2.children
+    body, = html.children
+    section, = body.children
+    div_1, = section.children
+    assert div_1.margin_top == 0
+    div_2, = div_1.children
+    assert div_2.margin_top == 5
+    assert div_2.content_box_y() == 5
+
+
 @assert_no_logs
 def test_page_names_1():
     pages = render_pages('''