Avoid infinite loops with td with break-inside: avoid

Related to #1547.

tests/layout/test_table.py

@@ -2753,3 +2753,21 @@ def test_table_break_children_margin():
       </table>
     '''
     assert len(render_pages(html)) == 3
+
+
+def test_table_td_break_inside_avoid():
+    # Test regression: https://github.com/Kozea/WeasyPrint/issues/1547
+    html = '''
+      <style>
+        @page { size: 4cm }
+        td { break-inside: avoid; line-height: 3cm }
+      </style>
+      <table>
+        <tr>
+          <td>
+            a<br>a
+          </td>
+        </tr>
+      </table>
+    '''
+    assert len(render_pages(html)) == 2

weasyprint/layout/table.py

@@ -157,20 +157,35 @@ def group_layout(group, position_y, bottom_space, page_is_empty,
                     cell.remove_decoration(start=True, end=False)
                     if table.style['border_collapse'] == 'collapse':
                         table.skip_cell_border_top = True
+
+                # First try to render content as if there was already something
+                # on the page to avoid hitting block_level_layout’s TODO. Then
+                # force to render something if the page is actually empty, or
+                # just draw an empty cell otherwise. See
+                # test_table_break_children_margin.
                 new_cell, cell_resume_at, _, _, _ = block_container_layout(
                     context, cell, bottom_space, cell_skip_stack,
                     page_is_empty=False, absolute_boxes=absolute_boxes,
                     fixed_boxes=fixed_boxes, adjoining_margins=None,
                     discard=False)
                 if new_cell is None:
-                    cell = cell.copy_with_children([])
-                    cell, _, _, _, _ = block_container_layout(
-                        context, cell, bottom_space, cell_skip_stack,
-                        page_is_empty=True, absolute_boxes=[], fixed_boxes=[],
-                        adjoining_margins=None, discard=False)
-                    cell_resume_at = {0: None}
+                    if page_is_empty:
+                        cell, cell_resume_at, _, _, _ = block_container_layout(
+                            context, cell, bottom_space, cell_skip_stack,
+                            page_is_empty=True, absolute_boxes=[],
+                            fixed_boxes=[], adjoining_margins=None,
+                            discard=False)
+                    else:
+                        cell = cell.copy_with_children([])
+                        cell, _, _, _, _ = block_container_layout(
+                            context, cell, bottom_space, cell_skip_stack,
+                            page_is_empty=True, absolute_boxes=[],
+                            fixed_boxes=[], adjoining_margins=None,
+                            discard=False)
+                        cell_resume_at = {0: None}
                 else:
                     cell = new_cell
+
                 cell.remove_decoration(
                     start=cell_skip_stack is not None,
                     end=cell_resume_at is not None)