settings: Add a saving-saved indicator to the 'SETTINGS/TOPICS' UI.

[prakhar1144]

#26016 (comment)

I think the settings "topics" UI is missing a loading...saved indicator interaction to show you that your changes are saved; right now you get no feedback at all that your change works. But that feels like something that'd be OK as an immediate follow-up.

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This commit adds a 'saving...' - 'saved' indicator to the 'SETTINGS/TOPICS' UI.

This improves the UX by reflecting that the changes are saved.

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Screenshots and screen captures:

screen-capture.webm

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[abhijeetbodas2001]

Thanks for the followup! Tested this out manually -- works great.
One question I have is, while we are waiting for the success response (and when we are showing the loading indicator), should we disable the dropdown selector? For example, see the below video, which seems like a weird experience, if you try to change the policy a second time before the first request is completed. (I introduced artificial latency by putting time.sleep(5) in the server code). It might be possible that this edge case is too rare and not worth working on, so I'm not sure.
Screencast from 2023-07-24 22-54-00.webm

[abhijeetbodas2001]

Actually we have the same "problem" (if we can call it a problem that is) in other settings panes as well. So probably this edge case is rare enough that we can ignore it :)

[prakhar1144]

@abhijeetbodas2001
I think since we never received any reports of such a weird experience, there are very, very rare chances.
But even if we plan to fix that, I guess doing it as a separate mini-project would be better. (Fixing it at all other places)

[timabbott]

This is great, though I'm a bit worried that this looks a bit misaligned vertically when next to the ? button; maybe a good topic to bring up in #frontend and figure out if there's something cleaner we could be doing.