Unexpected behavior of the HTML repr of meta-estimators

Here's a list of some unexpected behaviors of the HTML repr of meta-estimators
- `Pipeline` doesn't display its named steps

  <img width="164" height="95" alt="Image" src="https://github.com/user-attachments/assets/c368188d-544f-4bb1-bfeb-d76490f5146a" />

  This was maybe intentional ? In comparison `FeatureUnion` does for instance

  <img width="282" height="91" alt="Image" src="https://github.com/user-attachments/assets/2a6ded8c-d148-4852-a03c-0a88c1b8bc49" />

- a `Pipeline` in a meta-estimator doesn't render properly; it doesn't have the dashed border
  
  <img width="186" height="120" alt="Image" src="https://github.com/user-attachments/assets/471ce468-8597-4fbc-b718-870c4f11a1fd" />

  Another meta-estimator renders properly

  <img width="294" height="116" alt="Image" src="https://github.com/user-attachments/assets/cb467dcd-63c2-4a54-b92a-d6686b3b3bc8" />

- transformers of `ColumnTransformer` are expandable to show the selected columns, but there's no additional info. I think it should be explicit that these are the selected columns.

  <img width="274" height="106" alt="Image" src="https://github.com/user-attachments/assets/e44d47fb-c784-4631-9a73-3d274671430e" />

  Note that this could be fixed by https://github.com/scikit-learn/scikit-learn/pull/31937

- When the inner estimator of a meta-estimator is not a meta-estimator itself, it's expandable but the dropdown is not useful anymore (it's the non-html repr of the estimator basically):

  <img width="169" height="129" alt="Image" src="https://github.com/user-attachments/assets/21723076-ae7e-46e3-8d22-3561aef1b1ec" />

  Now that we have the parameter table, it's not useful anymore to have the additional repr which is redundant and less informative. I'd be in favor of removing the dropdown

- When the inner estimator of a meta-estimator is a meta-estimator itself, it's expandable but sometimes there's no dropdown or the dropdown is the non-html repr of the meta-estimator, and there's no parameter table.

  <img width="178" height="121" alt="Image" src="https://github.com/user-attachments/assets/4fbdb1fa-8f5e-4d32-9bd2-56bc4e89d891" />

  <img width="392" height="143" alt="Image" src="https://github.com/user-attachments/assets/1294de22-afcb-49c3-9256-b0cda06f352a" />

  I think that it should display the parameter table instead in both cases

COMMENT:
>Pipeline doesn't display its named steps
This was maybe intentional ? In comparison FeatureUnion does for instance

No, it's an issue that needs to be fixed (not introduced by the PRs for params table, etc).

>a Pipeline in a meta-estimator doesn't render properly; it doesn't have the dashed border

You are right, it doesn't. And it was like that before the param table work. For example commit "1527b1fe98 DOC Rework voting classifier example" already has the issue. That PR that was merged earlier.

>transformers of ColumnTransformer are expandable to show the selected columns, but there's no additional info. I think it should be explicit that these are the selected columns.
Note that this could be fixed by https://github.com/scikit-learn/scikit-learn/pull/31937

Yes (I hope).

>When the inner estimator of a meta-estimator is not a meta-estimator itself, it's expandable but the dropdown is not useful anymore (it's the non-html repr of the estimator basically):
Now that we have the parameter table, it's not useful anymore to have the additional repr which is redundant and less informative. I'd be in favor of removing the dropdown

Yes, I think Guillaume suggested to keep it when I asked.

The last point:
>I think that it should display the parameter table instead in both cases

Yes, I need to figure out why. I had not seen this one.

Labels: Documentation, frontend