Handle more cases of margin-break use cases

This solution is not perfect, but at least it works for table cells (whose
top margins never collapse) and avoids the related workaround.

Related to #1547.

weasyprint/layout/block.py

@@ -30,15 +30,20 @@ def block_level_layout(context, box, bottom_space, 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
-            # collapsing margins 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':
-                if not context.forced_break:
+        if context.current_page > 1 and page_is_empty:
+            # TODO: this condition is wrong, it only works for blocks whose
+            # parent breaks collapsing margins. It should work for blocks whose
+            # one of the ancestors breaks collapsing margins.
+            # See test_margin_break_clearance.
+            collapse_with_page = (
+                containing_block.is_for_root_element or
+                adjoining_margins)
+            if collapse_with_page:
+                if box.style['margin_break'] == 'discard':
                     box.margin_top = 0
+                elif box.style['margin_break'] == 'auto':
+                    if not context.forced_break:
+                        box.margin_top = 0
 
         collapsed_margin = collapse_margin(
             adjoining_margins + [box.margin_top])

weasyprint/layout/table.py

@@ -165,24 +165,17 @@ def group_layout(group, position_y, bottom_space, page_is_empty,
                 # test_table_break_children_margin.
                 new_cell, cell_resume_at, _, _, _ = block_container_layout(
                     context, cell, bottom_space, cell_skip_stack,
-                    page_is_empty=False, absolute_boxes=absolute_boxes,
+                    page_is_empty=page_is_empty, absolute_boxes=absolute_boxes,
                     fixed_boxes=fixed_boxes, adjoining_margins=None,
                     discard=False)
                 if new_cell is None:
-                    if page_is_empty:
-                        cell, cell_resume_at, _, _, _ = block_container_layout(
-                            context, cell, bottom_space, cell_skip_stack,
-                            page_is_empty=True, absolute_boxes=[],
-                            fixed_boxes=[], adjoining_margins=None,
-                            discard=False)
-                    else:
-                        cell = cell.copy_with_children([])
-                        cell, _, _, _, _ = block_container_layout(
-                            context, cell, bottom_space, cell_skip_stack,
-                            page_is_empty=True, absolute_boxes=[],
-                            fixed_boxes=[], adjoining_margins=None,
-                            discard=False)
-                        cell_resume_at = {0: None}
+                    cell = cell.copy_with_children([])
+                    cell, _, _, _, _ = block_container_layout(
+                        context, cell, bottom_space, cell_skip_stack,
+                        page_is_empty=True, absolute_boxes=[],
+                        fixed_boxes=[], adjoining_margins=None,
+                        discard=False)
+                    cell_resume_at = {0: None}
                 else:
                     cell = new_cell