Don't crash when fixed boxes are nested

Fix #728.

weasyprint/layout/__init__.py

@@ -20,7 +20,7 @@
 
 from collections import defaultdict
 
-from .absolute import absolute_box_layout
+from .absolute import absolute_box_layout, absolute_layout
 from .backgrounds import layout_backgrounds
 from .pages import make_all_pages, make_margin_boxes
 from ..formatting_structure import boxes
@@ -85,13 +85,20 @@ def layout_fixed_boxes(context, pages, containing_page):
         for box in page.fixed_boxes:
             # As replaced boxes are never copied during layout, ensure that we
             # have different boxes (with a possibly different layout) for
-            # each pages
+            # each pages.
             if isinstance(box, boxes.ReplacedBox):
                 box = box.copy()
-            # Use an empty list as last argument because the fixed boxes in the
-            # fixed box has already been added to page.fixed_boxes, we don't
-            # want to get them again
-            yield absolute_box_layout(context, box, containing_page, [])
+            # Absolute boxes in fixed boxes are rendered as fixed boxes'
+            # children, even when they are fixed themselves.
+            absolute_boxes = []
+            yield absolute_box_layout(
+                context, box, containing_page, absolute_boxes)
+            while absolute_boxes:
+                new_absolute_boxes = []
+                for box in absolute_boxes:
+                    absolute_layout(
+                        context, box, containing_page, new_absolute_boxes)
+                absolute_boxes = new_absolute_boxes
 
 
 def layout_document(enable_hinting, style_for, get_image_from_uri, root_box,

weasyprint/tests/test_layout/test_position.py

@@ -328,7 +328,6 @@ def test_fixed_positioning_regression_1():
         @page { size: 200px 100px; margin: 0 }
         article { break-after: page }
         .fixed { position: fixed; top: 10px; width: 20px }
-        ul {  }
       </style>
       <ul class="fixed" style="right: 0"><li>a</li></ul>
       <img class="fixed" style="right: 20px" src="pattern.png" />
@@ -353,3 +352,38 @@ def test_fixed_positioning_regression_1():
     assert (div.position_x, div.position_y) == (140, 10)
     assert (article.position_x, article.position_y) == (0, 0)
     assert 160 < ul.children[0].outside_list_marker.position_x < 170
+
+
+@assert_no_logs
+def test_fixed_positioning_regression_2():
+    # Regression test for https://github.com/Kozea/WeasyPrint/issues/728
+    page_1, page_2 = parse('''
+      <style>
+        @page { size: 100px 100px }
+        section { break-after: page }
+        .fixed { position: fixed; top: 10px; left: 15px; width: 20px }
+      </style>
+      <div class="fixed">
+        <article class="fixed" style="top: 20px">
+          <header class="fixed" style="left: 5px"></header>
+        </article>
+      </div>
+      <section></section>
+      <pre></pre>
+    ''')
+    html, = page_1.children
+    body, = html.children
+    div, section = body.children
+    assert (div.position_x, div.position_y) == (15, 10)
+    article, = div.children
+    assert (article.position_x, article.position_y) == (15, 20)
+    header, = article.children
+    assert (header.position_x, header.position_y) == (5, 10)
+
+    html, = page_2.children
+    div, body, = html.children
+    assert (div.position_x, div.position_y) == (15, 10)
+    article, = div.children
+    assert (article.position_x, article.position_y) == (15, 20)
+    header, = article.children
+    assert (header.position_x, header.position_y) == (5, 10)