REGR: <th> tags for notebook display closes #28204

[simonjayhawkins]

• closes CSS selector for index cells #28204
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[simonjayhawkins]

not added whatsnew. target 0.25.2?

[TomAugspurger]

Was it a regression in 0.25 or earlier?

[simonjayhawkins]

haven't checked thoroughly but i think the bug was introduced in 0.25.1 #27991

[TomAugspurger]

With 0.25.0

In [2]: df = pd.DataFrame({"a": 1}, index=pd.MultiIndex.from_product([['a'], [2, 3]]))

In [3]: print(df._repr_html_())

<div>
<style scoped>
    .dataframe tbody tr th:only-of-type {
        vertical-align: middle;
    }

    .dataframe tbody tr th {
        vertical-align: top;
    }

    .dataframe thead th {
        text-align: right;
    }
</style>
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th></th>
      <th>a</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th rowspan="2" valign="top">a</th>
      <th>2</th>
      <td>1</td>
    </tr>
    <tr>
      <th>3</th>
      <td>1</td>
    </tr>
  </tbody>
</table>
</div>

and with 0.25.1

In [3]: print(df._repr_html_())
<div>
<style scoped>
    .dataframe tbody tr th:only-of-type {
        vertical-align: middle;
    }

    .dataframe tbody tr th {
        vertical-align: top;
    }

    .dataframe thead th {
        text-align: right;
    }
</style>
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th></th>
      <th>a</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td rowspan="2" valign="top">a</td>
      <td>2</td>
      <td>1</td>
    </tr>
    <tr>
      <td>3</td>
      <td>1</td>
    </tr>
  </tbody>
</table>
</div>

So it seems to have been a 0.25.1 regression (so target for 0.25.2)

[simonjayhawkins]

thanks @TomAugspurger . will do.

[TomAugspurger]

LGTM. Merge conflict in the release notes.