*   Bring back the feature that allows loading external route files from the router.

    This feature existed back in 2012 but got reverted with the incentive that
    https://github.com/rails/routing_concerns was a better approach. Turned out
    that this wasn't fully the case and loading external route files from the router
    can be helpful for applications with a really large set of routes.
    Without this feature, application needs to implement routes reloading
    themselves and it's not straightforward.

    ```ruby
    # config/routes.rb

    Rails.application.routes.draw do
      draw(:admin)
    end

    # config/routes/admin.rb

    get :foo, to: 'foo#bar'
    ```

    *Yehuda Katz*, *Edouard Chin*

*   Fix system test driver option initialization for non-headless browsers.

    *glaszig*

*   `redirect_to.action_controller` notifications now include the `ActionDispatch::Request` in
    their payloads as `:request`.

    *Austin Story*

*   `respond_to#any` no longer returns a response's Content-Type based on the
    request format but based on the block given.

    Example:

    ```ruby
      def my_action
        respond_to do |format|
          format.any { render(json: { foo: 'bar' }) }
        end
      end

      get('my_action.csv')
    ```

    The previous behaviour was to respond with a `text/csv` Content-Type which
    is inaccurate since a JSON response is being rendered.

    Now it correctly returns a `application/json` Content-Type.

    *Edouard Chin*

*   Replaces (back)slashes in failure screenshot image paths with dashes.

    If a failed test case contained a slash or a backslash, a screenshot would be created in a
    nested directory, causing issues with `tmp:clear`.

    *Damir Zekic*

*   Add `params.member?` to mimic Hash behavior.

    *Younes Serraj*

*   `process_action.action_controller` notifications now include the following in their payloads:

    * `:request` - the `ActionDispatch::Request`
    * `:response` - the `ActionDispatch::Response`

    *George Claghorn*

*   Updated `ActionDispatch::Request.remote_ip` setter to clear set the instance
    `remote_ip` to `nil` before setting the header that the value is derived
    from.

    Fixes #37383.

    *Norm Provost*

*   `ActionController::Base.log_at` allows setting a different log level per request.

    ```ruby
    # Use the debug level if a particular cookie is set.
    class ApplicationController < ActionController::Base
      log_at :debug, if: -> { cookies[:debug] }
    end
    ```

    *George Claghorn*

*   Allow system test screen shots to be taken more than once in
    a test by prefixing the file name with an incrementing counter.

    Add an environment variable `RAILS_SYSTEM_TESTING_SCREENSHOT_HTML` to
    enable saving of HTML during a screenshot in addition to the image.
    This uses the same image name, with the extension replaced with `.html`

    *Tom Fakes*

*   Add `Vary: Accept` header when using `Accept` header for response

    For some requests like `/users/1`, Rails uses requests' `Accept`
    header to determine what to return. And if we don't add `Vary`
    in the response header, browsers might accidentally cache different
    types of content, which would cause issues: e.g. javascript got displayed
    instead of html content. This PR fixes these issues by adding `Vary: Accept`
    in these types of requests. For more detailed problem description, please read:

    https://github.com/rails/rails/pull/36213

    Fixes #25842.

    *Stan Lo*

*   Fix IntegrationTest `follow_redirect!` to follow redirection using the same HTTP verb when following
    a 307 redirection.

    *Edouard Chin*

*   System tests require Capybara 3.26 or newer.

    *George Claghorn*

*   Reduced log noise handling ActionController::RoutingErrors.

    *Alberto Fernández-Capel*

*   Add DSL for configuring HTTP Feature Policy.

    This new DSL provides a way to configure an HTTP Feature Policy at a
    global or per-controller level. Full details of HTTP Feature Policy
    specification and guidelines can be found at MDN:

    https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Feature-Policy

    Example global policy:

    ```ruby
    Rails.application.config.feature_policy do |f|
      f.camera      :none
      f.gyroscope   :none
      f.microphone  :none
      f.usb         :none
      f.fullscreen  :self
      f.payment     :self, "https://secure.example.com"
    end
    ```

    Example controller level policy:

    ```ruby
    class PagesController < ApplicationController
      feature_policy do |p|
        p.geolocation "https://example.com"
      end
    end
    ```

    *Jacob Bednarz*

*   Add the ability to set the CSP nonce only to the specified directives.

    Fixes #35137.

    *Yuji Yaginuma*

*   Keep part when scope option has value.

    When a route was defined within an optional scope, if that route didn't
    take parameters the scope was lost when using path helpers. This commit
    ensures scope is kept both when the route takes parameters or when it
    doesn't.

    Fixes #33219.

    *Alberto Almagro*

*   Added `deep_transform_keys` and `deep_transform_keys!` methods to ActionController::Parameters.

    *Gustavo Gutierrez*

*   Calling `ActionController::Parameters#transform_keys`/`!` without a block now returns
    an enumerator for the parameters instead of the underlying hash.

    *Eugene Kenny*

*   Fix strong parameters blocks all attributes even when only some keys are invalid (non-numerical).
    It should only block invalid key's values instead.

    *Stan Lo*


Please check [6-0-stable](https://github.com/rails/rails/blob/6-0-stable/actionpack/CHANGELOG.md) for previous changes.