Avoid endless loops in grids

tests/layout/test_grid.py

@@ -2,6 +2,8 @@
 
 from math import isclose
 
+import pytest
+
 from ..testing_utils import assert_no_logs, render_pages
 
 
@@ -1863,3 +1865,77 @@ def test_grid_in_flex_after_full_height():
     grid, = flex.children
     grid_item, = grid.children
     assert not grid_item.children
+
+
+@assert_no_logs
+def test_grid_in_columns():
+    # Regression test for #2691.
+    page1, page2, = render_pages('''
+      <style>
+        @page { size: 12cm }
+        section { height: 10cm }
+      </style>
+      <div style="columns: 2">
+        <div style="display: grid">
+          <div>
+            <section></section>
+            <section></section>
+            <section></section>
+          </div>
+        </div>
+      </div>
+    ''')
+    html, = page1.children
+    body, = html.children
+    div, = body.children
+    column1, column2 = div.children
+    grid, = column1.children
+    assert len(grid.children) == 1
+    grid, = column2.children
+    assert len(grid.children) == 1
+
+    html, = page2.children
+    body, = html.children
+    div, = body.children
+    column1, column2 = div.children
+    grid, = column1.children
+    assert len(grid.children) == 1
+    grid, = column2.children
+    assert len(grid.children) == 0
+
+
+@pytest.mark.xfail
+@assert_no_logs
+def test_grid_in_columns_with_break():
+    # Regression test for #2695.
+    page1, page2, = render_pages('''
+      <style>
+        @page { size: 12px }
+        section { height: 4px }
+      </style>
+      <div style="columns: 1">
+        <div style="display: grid; margin-top: 6px">
+          <div>
+            <section></section>
+            <section></section>
+            <section></section>
+          </div>
+        </div>
+      </div>
+    ''')
+    html, = page1.children
+    body, = html.children
+    div, = body.children
+    column1, = div.children
+    grid, = column1.children
+    section1, = grid.children
+    assert section1.position_y == 6
+
+    html, = page2.children
+    body, = html.children
+    div, = body.children
+    column1, = div.children
+    grid, = column1.children
+    section2, section3 = grid.children
+    assert section2.position_y == 0
+    assert section3.position_y == 4

weasyprint/layout/grid.py

@@ -1315,7 +1315,6 @@ def _add_page_children(max_row=inf):
             context, child, bottom_space, child_skip_stack, parent,
             page_is_empty, absolute_boxes, fixed_boxes)[:3]
         if new_child:
-            page_is_empty = False
             broken_child = False
             span = _get_span(child.style['grid_row_start'])
             if child_resume_at:
@@ -1337,7 +1336,10 @@ def _add_page_children(max_row=inf):
                 # last child.
                 assert isinstance(new_child, boxes.ParentBox)
                 previous_skip_child = max(child_skip_stack) if child_skip_stack else 0
-                resume_at[y][i] = {previous_skip_child + len(new_child.children): None}
+                if not page_is_empty:
+                    resume_at[y][i] = {
+                        previous_skip_child + len(new_child.children): None}
+            page_is_empty = False
         else:
             if resume_at is None:
                 resume_at = {}
@@ -1418,10 +1420,10 @@ def _add_page_children(max_row=inf):
             broken_child = y + span >= next_page_last_row + 1
             if broken_child and child.style['box_decoration_break'] != 'clone':
                 child.remove_decoration(start=False, end=True)
-            child.height = (
+            child.height = max(0, (
                 context.page_bottom - bottom_space - child.position_y -
                 child.margin_top - child.border_top_width - child.padding_top -
-                child.margin_bottom - child.border_bottom_width - child.padding_bottom)
+                child.margin_bottom - child.border_bottom_width - child.padding_bottom))
             if broken_child:
                 # Child not fully drawn, keep advancement.
                 advancements[x, y] = child.margin_height()