The the updated prototype has been completed with the changes discussed.

In the process, we decided to go with solution #3 for T147351: Design a way to help users understand the difference between Highlighting and Filtering, and to use both effectively, since the drawbacks of a separate highlight menu (as in solution #2) became more apparent as we reviewed the the initial iterations on the prototype.

I added the info to the ticket description with a quick screencast to illustrate some of the changes.

I've played with the prototype.

Do the following action is illogical to me:

1. select various filters
2. find somewhere it is possible to highlight them (I've searched because I knew it was possible), on the top of the list
3. activate highlights
4. go back and find on the list the choices I've made before (long list, no highlight until you guess it is possible to click on the filter to go to it on the list)

Not easy.

The problem of filtering and then highlighting is complicated. If we want to have filtering first, shouldn't give the opportunity of highlighting when 2 or more filters are selected?

And still that melted colors issue.

Thanks for the input @Trizek-WMF

There is still an open question about how much prominent should "highlighting" be. As an auxiliary layer of information you may not need it for simple cases where you want the list of contributions that meet a given criteria.

There are several options to make the highlighting option more prominent and easier to reach such as making the header area of the filter panel to become sticky. That however comes at a price of leaving less room for the filters which are the main element in most cases.

There is also a bug that made the panel to remain at the previous scroll position when you close and reopen it, which was making the highlight option to go out of sight much easily. That is now fixed in the prototype.

The problem of filtering and then highlighting is complicated. If we want to have filtering first, shouldn't give the opportunity of highlighting when 2 or more filters are selected?

We want to support also the case of highlighting based on criteria that you are not filtering. For example, you should be able to filter for damaging edits and additionally highlight those cases where editors are newcomers but viewing also results of damaging edits made by more experienced users.
Given that it is complicated to anticipate the ways filters and highlights may be combined (and communicate that in an effective way) we opted for providing highlights as an optional layer that is secondary but independent from filtering.

And still that melted colors issue.

We are watching close in our new research round to see how T147295 helps clarifying things or not.

Although we continue to touch up the prototype, the prototype in general is in a state that feels relatively complete—and it's posted on a wide variety of tickets. So I'm closing this task. Feel free to reopen if you object.