IPE is entirely broken

I had the same problem and got the following error:

PHP Fatal error:  Declaration of Drupal\panels_ipe\Form\PanelsIPEBlockContentForm::actions(array $form, Drupal\Core\Form\FormStateInterface $form_state) must be compatible with Drupal\block_content\BlockContentForm::actions(array $form, Drupal\Core\Form\FormStateInterface $form_state): array in /Library/WebServer/Documents/dedicon-vookam/web/modules/contrib/panels/panels_ipe/src/Form/PanelsIPEBlockContentForm.php on line 18

So fixing the function declaration solved the problem for me.

Did a manual test to ensure this was fixed (screenshots to show it's not broken anymore)

Place a block:

Set it in a 2 column layout:

Login as another user to see it locked:

These two fixes made this way smaller of a change:

• #3549737: TypeError: $wrapper[method] is not a function (thanks @oulalahakabu). Cause: every IPE form used '#ajax' => ['method' => 'replace'], but jQuery has no .replace() method (it's .replaceWith()). Core's AJAX insert command threw before attachBehaviors ran, so the response was never applied and nothing was placed. This is the fix that makes blocks/layouts actually appear.
• #3465159: Error: Call to a member function getParameter() on null in Drupal\panels_ipe\Form\PanelsIPEBlockPluginForm->getContextsForPath() (line 505 of /var/www/docroot/modules/contrib/panels/panels_ipe/src/Form/PanelsIPEBlockPluginForm.php) (thanks @luk.tc, @inascon, @f2boot, @joseph.olstad, @anairamzap, @mvonfrie). Page Manager RC3+ renamed its route params to _page_manager_page/_page_manager_page_variant; the old code fatally errored when adding a block. Also covers IPE used without Page Manager.
• The earlier PanelsIPEBlockContentForm::actions(): array return-type fix on this branch (thanks @inascon).

AI helped me plow through the aged isssues to get this done, so also help to LLMs.

What this issue/MR fixed on top of those, to make it fully work + keep the tests green:

• CategoryView AJAX accounting the form loader reset ajax.ajaxing = false early, which defeated core's ajaxComplete suppression and left drupalActiveXhrCount unbalanced (test teardown reported "Unfinished AJAX requests").
• Uncached IPE build (#cache max-age 0) the editor render depends on the shared tempstore lock, which any user can change; without this, a second user was served a stale cached render and saw the editor as editable when it was actually locked.
• Functional test helpers getWebDriverSession()->accept_alert() → alert()->accept() for W3C WebDriver, and addBlock() now waits for the async-placed [data-block-id] before asserting.