Copy boxes when using them in stacking contexts

If we don’t copy the boxes, rendering pages twice will fail because creating
stacking contexts can change the boxes’ children.

Fix #1473.

tests/test_api.py

@@ -975,3 +975,12 @@ def gzip_compress(data):
         assert HTML(root_url + '/deflate').etree_element.get('test') == 'ok'
         assert HTML(
             root_url + '/raw-deflate').etree_element.get('test') == 'ok'
+
+
+@assert_no_logs
+def test_page_copy_relative():
+    # Regression test for https://github.com/Kozea/WeasyPrint/issues/1473
+    document = FakeHTML(string='<div style="position: relative">a').render()
+    duplicated_pages = document.copy([*document.pages, *document.pages])
+    pngs = duplicated_pages.write_png(split_images=True)
+    assert pngs[0] == pngs[1]

weasyprint/stacking.py

@@ -136,7 +136,7 @@ def dispatch_children(box):
                 result = dispatch(child)
                 if result is not None:
                     new_children.append(result)
-            box.children = new_children
+            box = box.copy_with_children(new_children)
             return box
 
         box = dispatch_children(box)