PCH Related Top Posts: Add filter values drop-down

[acicovic]

Description

This PR introduces the ability to select a specific Tag, Section (Category) or Author value as a way to fine-grain results within the PCH's Related Top Posts panel. Once one of the filter types (let's say Tag) has been selected and the post has more than 1 tags, a second drop-down will be displayed to select the specific tag to filter by.

• Newly selected tags, categories and author in the Post Editor will be shown immediately as they're getting added/modified, without the need to save the post.
• Filter types that aren't available (for example when a post doesn't have tags or categories) are not shown as options in the UI.
• When only a single filter type or value is available, it automatically gets selected without displaying any drop-downs.
• WordPress Pages don't have tags or categories by design, so they are limited to the author.
• The filter value drop-down only contains the values that are pertinent to the current post. For example, it won't show all the website's tags, but only the tags that are assigned to the specific post. It is also searchable to accommodate for many entries.
• This implementation doesn't current support the Co-Authors Plus plugin. This possibility will be investigated in the future.

Motivation and context

• Completes the work that was started in PCH Related Top Posts: Add filter type drop-down #1916 and closes PCH Related Top Posts: Allow user to choose meta filter(s) #940.
• This work is based on Implement a base endpoint class #1976 which should be reviewed and merged first.

How has this been tested?

Some existing tests were slightly modified to pass. The testing was manual and there are no automated tests specifically targeting filter type and value selection.

Screenshots

The filter type drop-down

Shown	Clicked

No category (section) assigned to the post	No tag or section, auto-fallback to author

The filter value drop-down

Post with many tags (collapsed)	Post with many tags (expanded)

The same view/functionality applies in the case of many categories or authors.

[vaurdan]

Looks good to me! :) I have added an inline comment with a question, but otherwise, no blockers from me.

Regarding the need to refresh to get co-authors, the problem is related with the fact that the CAP terms are only added to the post on save, and therefore the new local API endpoint returns no co-authors, making the filter either empty or outdated, right?

[acicovic]

Regarding the need to refresh to get co-authors, the problem is related with the fact that the CAP terms are only added to the post on save, and therefore the new local API endpoint returns no co-authors, making the filter either empty or outdated, right?

I haven't dug in detail about why this is exactly happening so I don't know, but it does feel quirky in regular use to the point I'm thinking of temporarily dropping Co-Authors support and directly getting the Author from Gutenberg. The final solution would be to get co-authors data from Gutenberg as well, as we can do with the other data.

[acicovic]

I'm dropping Co-Authors support for now, to make the user experience better. We can then sort out in time how to get the data from the plugin.

[acicovic]

I've removed Co-Authors Plus support, and modified the PR's description to reflect that.

[vaurdan]

Looks great! 👍