Copy columns when copying tables

Backgrounds set on a column are applied to the column cells, and these cells
depend on the page we’re on. If we use the same column object for all pages,
the corresponding cells are only the ones of the last page.

Fix #2296.

tests/draw/test_table.py

@@ -1620,3 +1620,36 @@ def test_tables_split_row(assert_pixels):
           <td>a<br>a</td>
         </tr>
       </table>''')
+
+
+@assert_no_logs
+def test_tables_column_background(assert_pixels):
+    # Regression test: https://github.com/Kozea/WeasyPrint/issues/2296
+    assert_pixels('''
+      KKKKKKKK
+      KRRBBRRK
+      KRRBBRRK
+      KKKKKKKK
+
+      KKKKKKKK
+      KRRBBRRK
+      KRRBBRRK
+      KKKKKKKK
+    ''', '''
+      <style>
+        @page { margin: 0; size: 8px 4px }
+        col { background: blue }
+        td { border: 1px solid black; color: red; vertical-align: top;
+             font-family: weasyprint; font-size: 2px; line-height: 1 }
+      </style>
+      <table>
+        <colgroup>
+          <col>
+        </colgroup>
+        <tr>
+          <td>a a</td>
+        </tr>
+        <tr>
+          <td>a a</td>
+        </tr>
+      </table>''')

weasyprint/layout/table.py

@@ -566,6 +566,8 @@ def table_layout(context, table, bottom_space, skip_stack, containing_block,
         ([header] if header is not None else []) +
         new_table_children +
         ([footer] if footer is not None else []))
+    table.column_groups = tuple(
+        column_group.deepcopy() for column_group in table.column_groups)
     remove_end_decoration = resume_at is not None and not has_footer
     table.remove_decoration(remove_start_decoration, remove_end_decoration)
     if collapse: