Address mystery of `fixed/showcase` front containers

[cemms1]

Background

As discussed in the Tools/Platforms catch up (20th Sept 2023), there is a gap in knowledge of how the fixed/showcase container works and when it is used.

It is specifically for Google Showcase, so that Google News receives updates in a specified format. Here's the Guardian on Google News: https://news.google.com/publications/CAAqBggKMJeqezDfswk

It should never be rendered on a normal front, but frontend does build and maintain this https://github.com/guardian/frontend/blob/main/common/app/layout/slices/FixedContainers.scala#L55, which is quite unlike Apple News which is maintained by the CoPip team (who also manage CAPI). It is not marked as "not-for-production" since it is for production; it's just never meant to be rendered directly on dotcom or in apps.

Solution

We should continue to support this container type, and ensure it is well documented in the code.

We should also ensure that DCR does not fall over in the process of rendering this, if mistakenly attempted.

[alinaboghiu]

PR that introduced showcase fronts is here: #24038

A showcase front is created in the fronts tool by choosing the showcase priority on the dashboard of the fronts tool:

The showcase containers are intended for use on the showcase front exclusively.

Cleanup Task

A showcase front should always be rendered with Frontend as it is not supported in DCR (and does not need to be). Currently, if we remove the allow list of "SUPPORTED_COLLECTIONS" from the Facia Picker, the code will attempt to render a showcase front with DCR.

We should introduce an extra check (same as we do for isRss) or better yet change the decision order in the Facia Controller, to ensure showcase fronts are always rendered with Frontend.

We should also make sure that if a showcase container is ever accidentally introduced on a regular non-showcase front, DCR is able to ignore it and now blow up.

Then we can remove the "SUPPORTED_COLLECTIONS" allow list from Facia Picker, since we will support all collections.

[cemms1]

Labelled as a "pick me up" for non-scheduled work.
Note that this is very low priority as it's a bit of an edge case