Don't display absolutes when their parents have no other children

If a parent box can only display absolute boxes at the bottom of the page, but
has other children that have to be drawn on the next page, keep the absolute
boxes for the next page too. It's at least very useful for markers.

Fix #945.

weasyprint/layout/blocks.py

@@ -569,6 +569,15 @@ def block_container_layout(context, box, max_position_y, skip_stack,
                         # else:
                         # ignore this 'avoid' and break anyway.
 
+                if all(child.is_absolutely_positioned()
+                       for child in new_children):
+                    # This box has only rendered absolute children, keep them
+                    # for the next page. This is for example useful for list
+                    # markers.
+                    remove_placeholders(
+                        new_children, absolute_boxes, fixed_boxes)
+                    new_children = []
+
                 if new_children:
                     resume_at = (index, None)
                     break

weasyprint/tests/test_layout/test_list.py

@@ -70,3 +70,33 @@ def test_lists_empty_item():
     unordered_list, = body.children
     li1, li2, li3 = unordered_list.children
     assert li1.position_y != li2.position_y != li3.position_y
+
+
+def test_lists_page_break():
+    # Regression test for https://github.com/Kozea/WeasyPrint/issues/945
+    page1, page2 = parse('''
+      <style>
+        @font-face { src: url(AHEM____.TTF); font-family: ahem }
+        @page { size: 300px 100px }
+        ul { font-size: 30px; font-family: ahem; margin: 0 }
+      </style>
+      <ul>
+        <li>a</li>
+        <li>a</li>
+        <li>a</li>
+        <li>a</li>
+      </ul>
+    ''')
+    html, = page1.children
+    body, = html.children
+    ul, = body.children
+    assert len(ul.children) == 3
+    for li in ul.children:
+        assert len(li.children) == 2
+
+    html, = page2.children
+    body, = html.children
+    ul, = body.children
+    assert len(ul.children) == 1
+    for li in ul.children:
+        assert len(li.children) == 2