T/4: Add debounce mechanism.

[jodator]

Suggested merge commit message (convention)

Type: Add debounce mechanism for requesting a feed. Closes ckeditor/ckeditor5#4619.

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Additional information

• This PR solves the problem of asynchronous server by:
  • Adding a debounce mechanism for requesting a feed (relatively short - 100ms so if the feed callback promise resolves within 100-150ms it should still be perceived as "fast". - note that toolbar plugin is debounced to 200ms.
  • Handling errors and request out-of-order problems by:
    • discarding all but last requested feed (the comparison i relatively simple - just remember the last requested feed (the text for it) and if the response is not for this feed discard it
    • showing warning notification for any errors that ocured in feed callback (most likely due to communication/connectivity problems)

[jodator]

@Mgsy As this is issue should be prone to manual tests please take a look at the new manual tests for asynchronous feeds in mentions (others should work too).

@mlewand What is done is in the description but I've added a small follow up: #50 that can be resolved after this PR.

[jodator]

Edge.... :(

[jodator]

🤞 last fixes to the test were pushed - hopefully will be OK now.

[jodator]

@Reinmar thanks for the insights. I've cleaned up the code a bit but still, some minor work is needed:

1. Add tests for updating a marker.
2. Check some ifs if they cannot have some better code (e.g. adding isInCompletionMode() check).
3. Document the events.

But if feel free to check the code again if you wish - I don't think that much will change :)

[jodator]

@Mgsy Could you please re-check this PR as before? This should work the same - ie no errors, etc. The code was refactored but it looks like everything works as expected.

@Reinmar So it is ready to review. The only thing that I've wanted to ask is should we expose events? I'm not sure if they are important to others. Right now they are documented but without much fuzz around them.

[jodator]

@mlewand the PR is OK now

[mlewand]

LGTM pushed couple minor details.

Note: 🙈 there might be 6 outdated comments, from months ago back when I was doing previous review. GH shows the count in rev summary window, but doesn't show the comments in the "Files changed" view so I can't remove them 🤔

[FilipTokarski]

Tested on all browsers, seems to work fine now 👍