Avoid float collision with box establishing formatting context

Related to #2019.

weasyprint/formatting_structure/boxes.py

@@ -313,8 +313,6 @@ def is_monolithic(self):
     def establishes_formatting_context(self):
         """Return whether this box establishes a block formatting context."""
         # See https://www.w3.org/TR/CSS2/visuren.html#block-formatting
-        # TODO: columns shouldn't be block boxes, this condition would then be
-        # useless when this is fixed
         return (
             self.is_floated() or
             self.is_absolutely_positioned() or

weasyprint/layout/block.py

@@ -124,8 +124,11 @@ def block_box_layout(context, box, bottom_space, skip_stack,
     result = block_container_layout(
         context, box, bottom_space, skip_stack, page_is_empty,
         absolute_boxes, fixed_boxes, adjoining_margins, discard, max_lines)
-    new_box = result[0]
-    if new_box and new_box.is_table_wrapper:
+    # TODO: columns shouldn't be block boxes, this condition would then be
+    # useless when this is fixed.
+    if not (new_box := result[0]) or new_box.is_column:
+        return result
+    if new_box.is_table_wrapper or new_box.establishes_formatting_context():
         # Don't collide with floats
         # https://www.w3.org/TR/CSS21/visuren.html#floats
         position_x, position_y, _ = avoid_collisions(

weasyprint/layout/float.py

@@ -195,12 +195,13 @@ def avoid_collisions(context, box, containing_block, outer=True):
     # - line boxes
     # - table wrappers
     # - block-level replaced box
-    # - element establishing new formatting contexts (not handled)
+    # - element establishing new formatting contexts
     assert (
         (box.style['float'] in ('right', 'left')) or
         isinstance(box, boxes.LineBox) or
         box.is_table_wrapper or
-        isinstance(box, boxes.BlockReplacedBox))
+        isinstance(box, boxes.BlockReplacedBox) or
+        box.establishes_formatting_context())
 
     # The x-position of the box depends on its type.
     position_x = max_left_bound