This is a followup to the discussion in #139524. Planned API changes means that more information can be surfaced on the UI (in particular, timestamp and thread status). At the same time, we want to reduce visual complexity; it can already be difficult to navigate through the set of comments and find which ones are relevant.

I'd like to explore here designs that remove intermediate replies in a discussion thread. Rationale: reading a full discussion in the comments panel is not really feasible (because comment snippets are truncated and lack styling). If someone wants to read the discussion, they should open it. We need to show enough context so that a user can find the thread they are interested in.

Design 1.

Here, we show only the first and the last messages in each thread. The first message is useful for the context. The last message helps the user know what is the state of the thread, and if there was a message since they last read it.

We could add "..." somewhere to make it clearer that intermediate messages are not shown.

Design 2.

Here, we don't show the first message and we focus on the last update. We give information about hidden messages: how many messages are not shown and who where the authors. This design makes it more obvious that the full thread is not shown.

Other thoughts:

• Being able to hide the resolved threads would be very useful. This would be similar to the filter we already have in the Problems panel (e.g. Show warnings can be unchecked).
• It would also help if we had control on which thread is expanded. The API only lets us expand/collapse a thread in the editor view; maybe we could have the same for the Comments panel? This feature is important IMO if we keep a design that lists all intermediate replies.

(mocks designed by @albertelo)
cc @alexr00 and @isidorn