From 2326bcc636e7dc3fda544f5cb7b6c58f0d94b010 Mon Sep 17 00:00:00 2001
From: Guillaume Ayoub <guillaume@courtbouillon.org>
Date: Sun, 20 Aug 2023 15:07:55 +0200
Subject: [PATCH] Define monolithic boxes

Related to #1936.
---
 weasyprint/formatting_structure/boxes.py | 10 ++++++++++
 weasyprint/layout/block.py               | 22 ++++++++++++----------
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/weasyprint/formatting_structure/boxes.py b/weasyprint/formatting_structure/boxes.py
index 9d8dcf250..64995b895 100644
--- a/weasyprint/formatting_structure/boxes.py
+++ b/weasyprint/formatting_structure/boxes.py
@@ -289,6 +289,16 @@ def is_in_normal_flow(self):
             self.is_floated() or self.is_absolutely_positioned() or
             self.is_running() or self.is_footnote())
 
+    def is_monolithic(self):
+        """Return whether this box is monolithic."""
+        # https://www.w3.org/TR/css-break-3/#monolithic
+        return (
+            isinstance(self, AtomicInlineLevelBox) or
+            isinstance(self, ReplacedBox) or
+            self.style['overflow'] in ('auto', 'scroll') or
+            (self.style['overflow'] == 'hidden' and
+             self.style['height'] != 'auto'))
+
     # Start and end page values for named pages
 
     def page_values(self):
diff --git a/weasyprint/layout/block.py b/weasyprint/layout/block.py
index b09afeafa..2e1dc9958 100644
--- a/weasyprint/layout/block.py
+++ b/weasyprint/layout/block.py
@@ -249,9 +249,11 @@ def _out_of_flow_layout(context, box, index, child, new_children,
         # New page if overflow
         page_overflow = context.overflows_page(
             bottom_space, new_child.position_y + new_child.height)
-        if box.style['overflow'] == 'hidden' and box.style['height'] != 'auto':
-            page_overflow = False
-        if (page_is_empty and not new_children) or not page_overflow:
+        add_child = (
+            (page_is_empty and not new_children) or
+            not page_overflow or
+            box.is_monolithic())
+        if add_child:
             new_child.index = index
             new_children.append(new_child)
         else:
@@ -501,19 +503,19 @@ def _in_flow_layout(context, box, index, child, new_children, page_is_empty,
                 new_child.content_box_y() + new_child.height)
             new_position_y = (
                 new_child.border_box_y() + new_child.border_height())
-            page_overflow = context.overflows_page(
+            content_page_overflow = context.overflows_page(
                 bottom_space, new_content_position_y)
-            if (box.style['overflow'] == 'hidden' and
-                    box.style['height'] != 'auto'):
-                page_overflow = False
-            if page_overflow and not page_is_empty_with_no_children:
+            border_page_overflow = context.overflows_page(
+                bottom_space, new_position_y)
+            can_break = not (
+                page_is_empty_with_no_children or box.is_monolithic())
+            if can_break and content_page_overflow:
                 # The child content overflows the page area, display it on the
                 # next page.
                 remove_placeholders(
                     context, [new_child], absolute_boxes, fixed_boxes)
                 new_child = None
-            elif not page_is_empty_with_no_children and context.overflows_page(
-                    bottom_space, new_position_y):
+            elif can_break and border_page_overflow:
                 # The child border/padding overflows the page area, do the
                 # layout again with a higher bottom_space value.
                 remove_placeholders(