Form preprocess not finding the right button

Tested on the last dev version.

There is still the bug.

I have uploaded a fix.

I have done tests modifying directly the native search block form to add submit button. www/core/modules/search/src/Form/SearchBlockForm.php:

    $form['keys'] = [
      '#type' => 'search',
      '#title' => $this->t('Search'),
      '#title_display' => 'invisible',
      '#size' => 15,
      '#default_value' => '',
      '#attributes' => ['title' => $this->t('Enter the terms you wish to search for.')],
    ];
    $form['keys']['test1'] = [
      '#type' => 'submit',
      '#value' => $this->t('test 1'),
    ];

    $form['actions'] = ['#type' => 'actions'];
    $form['actions']['test 2'] = [
      '#type' => 'submit',
      '#value' => $this->t('test 2'),
    ];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Search'),
      // Prevent op from showing up in the query string.
      '#name' => '',
    ];

bootstrap_button_1.png: the result without the patch.
bootstrap_button_2.png: the result with the patch.
bootstrap_button_3.png: the result with the patch and with the "test 1" button commented out.

Thanks for the review.

Thanks @markcarver for the review.

Here is a new patch with your suggestion.

About the deprecation, I don't think it will be very frequent to have custom theme using the findButton method of the processManager. And maybe putting a warning in the next release note and maybe a change record will be good.

OK... Now I see how you handle deprecation.

Here is a new patch.

Thanks for the review.

Thanks for the commit :)