[regression] After placing a block and saving, you are directed to the block library (example: admin/structure/block/library/bartik) rather than to the block layout

A bit weird for sure. This patch should fix that.

A nicer implementation

This appears in 8.2 and exists in 8.3.

#2745911: Block add links should respect destination

The problem seems to be the usage of RedirectDestination::get(). This method always return a result even if the 'destination' query string is not set:

  /**
   * {@inheritdoc}
   */
  public function get() {
    if (!isset($this->destination)) {
      $query = $this->requestStack->getCurrentRequest()->query;
      if (UrlHelper::isExternal($query->get('destination'))) {
        $this->destination = '/';
      }
      elseif ($query->has('destination')) {
        $this->destination = $query->get('destination');
      }
      else {
        $this->destination = $this->urlGenerator->generateFromRoute('<current>', [], ['query' => UrlHelper::buildQuery(UrlHelper::filterQueryParameters($query->all()))]);
      }
    }

    return $this->destination;
  }

so if the destination is not set the current route path is returned.
Probably you can use the value retrieved from the request instead of rely on redirect destination service

Added a test for ensure that if destination query string is not present you are redirected by default to the block list page

Honestly I would consider this a bug in RedirectDestination::get(), but I don't think we can change that behavior without breaking BC, so I agree this is a good fix.

We should probably document that RedirectDestination::get() will ALWAYS return something, even if it's the current page.

+1 to this.

I copied @return comment from RedirectDestinationInterface::getAsArray() to ::get(). Not sure if further documentation is needed.

Given the regression, I think we should actually revert the patch and then fix stuff without introducing a regression in the first place. This could easily affect other code as well.

Marked #2745911: Block add links should respect destination for revert.
Let's correctly fix it again from scratch.