The name was vestigial. Rename it to `_bucketsContainer` to be more
meaningful and indicate that it is private. Several other private fields
have also been prefixed accordingly.

When a PDF page transitions from a rendered to non-rendered state the
text layer element is removed and existing highlights in that layer
briefly become disconnected from the document. As a result they have an
empty/zero client rect as returned by `element.getBoundingClientRect()`.
This caused the bucket-building logic to think that the anchor
had moved to the top of the screen as the `top` and `bottom` coordinates
of the anchor box became zero. This could lead to anchors which should
be in the "below screen" bucket briefly jumping to the "above screen"
bucket. Once re-anchoring completes and the highlight is anchored into a
placeholder element for the non-rendered page, its anchor box becomes
non-empty again and the anchor "jumps back" to the "below screen" box.

This commit adds logic to ignore any highlights which have zero height,
including highlight elements that are disconnected from the document.

Co-authored-by: Robert Knight <robertknight@gmail.com>

Delegator does no longer exits.

The new comment explains the source of the events and its final
destination.

A few lines have been removed as a result of the code simplification.

If the user hovered an annotation card before anchoring in the document
completed, or if an annotation was re-anchored in a PDF after the card
was hovered, then the highlight in the document was incorrectly shown in
an un-focused rather than focused state.

Fix the issue by keeping track of the current set of focused annotations
in the Guest and setting the focus state correctly for new highlights
added by `anchor`.

Part of https://github.com/hypothesis/client/issues/3269

 - Revise the documentation for `createPlaceholder` to better explain
   what a placeholder is and the 1:N relationship between placeholders
   and highlights for a given page. Also clarify what the "approximate
   location" means

 - Rename a variable `isPlaceholder` => `inPlaceholder` as this is more
   accurate

Fix incorrect result if the passed node has no parent.

Several different parts of the code need to know about placeholder
elements used to anchor annotations that refer to not-yet-rendered pages
of a document in a PDF. Centralize the logic for creating placeholders
and testing whether a node is inside a placeholder in one module.

A trivial changes it that the "Loading annotations" text now ends with
an ASCII "..." instead of unicode ellipsis "…". This is easier to type
since the text is not intended to be seen by the user.

Move the logic for scrolling the content to the highlights associated
with an anchor from `Guest` to the content-type specific integration
(`HTMLIntegration`, `PDFIntegration`). The implementations of this logic
in `HTMLIntegration` and `PDFIntegration` are currently the same as
before, but in future this will allow adapting the scrolling logic to
the content type.

In the case of PDFs the logic will need to allow for the fact that an
annotation may be re-anchored to a different element as the document is
scrolled due to the page containing the annotation changing between an
un-rendered and rendered state.

 - Add `scrollToAnchor` method to `Integration` interface and implement
   it for `HTMLIntegration`, `PDFIntegration`
 - Replace `scrollIntoView` call in guest with a call to the active
   integration's `scrollToAnchor` method

Bumps [katex](https://github.com/KaTeX/KaTeX) from 0.13.2 to 0.13.3.
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- [Changelog](https://github.com/KaTeX/KaTeX/blob/master/CHANGELOG.md)
- [Commits](https://github.com/KaTeX/KaTeX/compare/v0.13.2...v0.13.3)
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Bumps [gulp-replace](https://github.com/lazd/gulp-replace) from 1.0.0 to 1.1.1.
- [Release notes](https://github.com/lazd/gulp-replace/releases)
- [Commits](https://github.com/lazd/gulp-replace/commits)
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Bumps [@babel/core](https://github.com/babel/babel/tree/HEAD/packages/babel-core) from 7.13.15 to 7.13.16.
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- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.13.16/packages/babel-core)
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Bumps [postcss](https://github.com/postcss/postcss) from 8.2.10 to 8.2.12.
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- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.2.10...8.2.12)
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Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.888.0 to 2.892.0.
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- [Changelog](https://github.com/aws/aws-sdk-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/aws/aws-sdk-js/compare/v2.888.0...v2.892.0)
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Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 18.5.2 to 18.5.3.
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Bumps [core-js](https://github.com/zloirock/core-js/tree/HEAD/packages/core-js) from 3.10.1 to 3.11.0.
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Bumps [eslint](https://github.com/eslint/eslint) from 7.24.0 to 7.25.0.
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Bumps [terser](https://github.com/terser/terser) from 5.6.1 to 5.7.0.
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Bumps [sass](https://github.com/sass/dart-sass) from 1.32.10 to 1.32.11.
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Bumps [puppeteer](https://github.com/puppeteer/puppeteer) from 8.0.0 to 9.0.0.
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Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 6.2.5 to 6.3.1.
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- [Commits](https://github.com/getsentry/sentry-javascript/compare/6.2.5...6.3.1)
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Bumps [redux](https://github.com/reduxjs/redux) from 4.0.5 to 4.1.0.
- [Release notes](https://github.com/reduxjs/redux/releases)
- [Changelog](https://github.com/reduxjs/redux/blob/master/CHANGELOG.md)
- [Commits](https://github.com/reduxjs/redux/compare/v4.0.5...v4.1.0)
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Part of https://github.com/hypothesis/client/issues/3298

Checking `typeof <var>` is only needed when the variable may not exist
(eg. some browser global).

Put the logic for getting the grant token in one place so it can be
better documented.

Also remove or simplify some other code slightly upon a second reading.

A call to `refreshAccessToken` is not the only way that the token can
change while waiting for a response to the `POST /api/token` endpoint.

The previous name was a vestige from the days of Angular 1.x in the
client. Use a clearer name instead.

Refactor and better document the `tokenGetter` method to make it easier
to follow:

 - Split some sub-steps out into helper methods
 - Add comments to call out the high-levels steps in the flow

Extract a helper from a long async method to shorten it.

Make several functions/methods in the service easier to follow by
refactoring promise chains to async/await.

 - Add tests for `scrollToAnnotation`

 - Add test for failed `getDocumentInfo` request to host page

Convert the `frameSync` service to an ES class and rewrite the
documentation that explains what the service does.

Some internal `frameSync` methods are still set up in the constructor
with closures. These can be converted to ES class methods in later
refactoring.

Part of https://github.com/hypothesis/client/issues/3298.

Chrome 89 printed the following warnings on the console:

```
shadow-root.js:67 [Violation] Added non-passive event listener to a scroll-blocking 'touchstart' event. Consider marking event handler as 'passive' to make the page more responsive
```

See also: https://www.chromestatus.com/feature/5745543795965952

I have removed a listener that it isn't no longer needed, because it is wrapped in
a shadow DOM that already stop the bubbling up of the event.

Another 'touchstart' listener had the potential to slow down scrolling
on the host page on mobile devices. To avoid that we have made listener
passive.

These changes remove the warnings without affecting the current
behaviour of the application.

Move the logic for retrying API metadata fetching into the `getJSON`
helper function so that it is used both when fetching the API index
(`/api`) and the service links (`/api/links`).