Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system.

Issue #2084463 by dawehner, pwolanin, David_Rothstein, tim.plunkett, Xano: Convert contextual links to a plugin system similar to local tasks/actions.

Issue #2048223 by dawehner, ParisLiakos, herom, fubhy, damiankloip, vijaycs85, joelpittet, tim.plunkett: Add $account argument to AccessCheckInterface::access() method and use the current_user() service.

Issue #2046367 by jhodgdon, amateescu, larowlan, juanolalla: Menu routing docs do not match what the code is doing.

Issue #2045931 by PerthSam152, LinL, adsw12, joelpittet: Replace all module_exists() deprecated function calls.

This reverts commit cc84f390 - bad
commit message.

Issue #2105167 by tim.plunkett: Fixed Legacy local actions are added alongside modern local actions.

Issue #2089351 by thedavidmeister, meba: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in core/includes.

Issue #2077473 by dawehner, damiankloip, pwolanin: Replace local action plugin discovery with YamlDiscovery and handle routes with parameters.

Issue #2100753 by Berdir: Fixed route_parameters break lazy-save logic in MenuLinkStorageController::save().

Issue #2070651 by pwolanin, dawehner, larowlan, tim.plunkett, ParisLiakos: Introduce a path-based breadcrumb builder, remove the one that's based on {menu_links()}.

Issue #1831846 by katbailey, maciej.zgadzaj, dawehner, RobLoach, moe4715, Hydra, larowlan, andypost: Fixed Help block is broken with language path prefixes.

Issue #2076283 by dawehner, pwolanin, neclimdul: Allow local task plugins on paths with a dynamic value (like a node).

Issue #2089635 by tim.plunkett, disasm, larowlan: Convert non-test non-form page callbacks to routes and controllers.

Issue #2084339 by alexpott: Fixed The configuration value system.maintenance.enabled should be moved to the state system.

Issue #2078855 by dawehner, pwolanin: Fixed Request::create() is slow, use a helper to duplicate instead.

Issue #2078583 by dawehner, pwolanin: Fixed menu callbacks removed from hook_menu() do not display any menus.

Issue #2078583 by dawehner, pwolanin | disasm: Fixed menu callbacks removed from hook_menu() do not display any menus.

Issue #1847002 by tim.plunkett, effulgentsia, msonnabaum: Move entity type classes from Drupal$provider\Plugin\Core\Entity to Drupal$provider\Entity.

Issue #2046385 by dawehner, pwolanin: Fixed _menu_navigation_links_rebuild() fails when menu_link module is not enabled.

Issue #2056895 by tim.plunkett, xjm: Fixed Routes with optional placeholders can fatal when placed in a menu.

Issue #2045453 by pwolanin, tim.plunkett, dawehner: Fixed menu_item_route_access() does not catch ResourceNotFoundException.

Issue #2045923 by drupalrv, tim.plunkett: Replace all module_invoke_all() deprecated function calls in procedural code.

Issue #2052389 by pwolanin: Fixed All elements added to the Request attributes should have a _ prepended unless they come from the path.

Issue #1589968 by cr0ss, Cottser, Pancho, DarthDrupal, Mark Carver: Move Title from process to preprocess, remove template_process_page().

Issue #2031473 by pwolanin, fubhy, dawehner, tim.plunkett: Convert menu local actions to plugins so that we can generate dynamic titles and paths.

Issue #1754246 by webflo, swentel, penyaskito, andypost, alexpott, YesCT, dagmar: Languages should be configuration entities.