This adoption simplifies the discovery of frames.

I did a little bit of clean-up in the tests: rearranged a few variables
alphabetically, and delete unneeded lines and variables.

This includes a fix for incorrect results under certain conditions when
the pattern length is an exact multiple of 32. See
https://github.com/robertknight/approx-string-match-js/pull/13.

This reverts commit 5d69a81c.

This reverts commit 2957944d.

This reverts commit 520b5c50.

In future PR, `Guest` will contain two bridges (like `FrameSync`).
Naming them by the destination frame will help identify the purpose of
it.

It seems that the convention for this `Bridge`s is to contain the word
`RPC` to indicate that it is a remote (inter-frame) procedure.

This adoption simplifies the discovery of frames.

I did a little bit of clean-up in the tests: rearranged a few variables
alphabetically, and delete unneeded lines and variables.

The tests crashed if either of these commands was executed:

- `yarn test --grep anchoring`, or
- `yarn test --grep 'anchoring/test/pdf|test/integration/anchoring'`

`src/anotator/anchoring/test/pdf-test.js` cleanup the Viewer by setting
the `window.PDFViewerApplication' to `null`. After that, if another test
created a `Guest` instance (like
`src/annotator/test/integration/anchoring-test.js`) will think that the
document is a PDF and will crash. This is the code that checks if the
document is a PDF:

https://github.com/hypothesis/client/blob/492ab6d56a5685356ca6b3a48644e047bb28dca0/src/annotator/integrations/pdf.js#L53-L56

The solution is either deleting `window.PDFViewerApplication' or setting
it to `undefined`.

We use `Window` objects as keys on a map. Windows can be removed.
`WeakMap` has the characteristic that keys that are claimed by the
garbage collector are removed from the map. In this way, we are memory
friendly and don't retain any defunct object.

Create a test page for PDF-based VS books, using content adapted from
https://bookshelf.vitalsource.com/reader/books/9781938168239/, an
OpenStax textboot that is available for free [1] in VitalSource.

 - Rename vitalsource.mustache => vitalsource-epub.mustache to make the
   distinction with the PDF example clearer.

 - Create vitalsource-pdf.mustache which serves as the "container" page
   for a PDF-based VS book. This is a copy of vitalsource-epub.mustache
   with the Prev/Next buttons removed and slight terminology changes
   ("chapter" => "page") to reflect how PDFs are handled in VS

 - Revise the descriptive text at the top of the VS PDF and EPUB
   examples to explain the purpose more clearly and relate them to how
   books are presented in the Bookshelf store, where the terms
   "reflowable" and "fixed" are used instead of "EPUB" and "PDF".

 - Add a very lightly edited copy of a real content page from a VS book
   in vitalsource-pdf-page.mustache. This is the HTML content from the
   iframe displaying a page in a PDF book in VS. The only changes I have
   made are small block of CSS customizations at the top (see comment)
   and a change to the URL of the rendered PDF page.

 - Update the links to the VitalSource test pages on the dev server.

[1] Even though the book is free, you have to add it to your VitalSource
    book library before you can access it. To do that, go to
    https://bookshelf.vitalsource.com and search for the book ID (9781938168239
    in this case), then click "Open Book".

Following these two comments:
* https://github.com/hypothesis/client/pull/3929/files#r750399422
* https://github.com/hypothesis/client/pull/3929/files#r750414036

I am proposing in this PR to remove the anti-collision field `authority`
from the message that is send to discover frames. I believe the message
contains enough information to avoid to be confused by other unrelated
`postMessage` in the host frame.

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

For PortFinder, I followed this advice:
https://github.com/hypothesis/client/pull/3881#discussion_r743030606,
except that I made the argument of `PorFinder#discover` and string
(instead of an object).

```
const portFinder = new PortFinder({ source: 'guest', hostFrame });
portFinder.discover('sidebar');
portFinder.discover('host');
```

For PortProvider, I followed this advice:
https://github.com/hypothesis/client/pull/3881#discussion_r743033013,
except that I used a getter.

```
cont portProvider = new PortProvider(...)
const bridge = new Bridge();
bridge.createChannel(portProvider.sidebarPort);
```

I have renamed the properties of `Message`:
- `source` becomes `authority`
- `channel` and `port` have been replaced by `frame1` and `frame2`

I did that to align `port1` and `frame1` and `port2` and `frame2`, while
also avoiding clashing with other names (`source`, `target`) which have
other meaning in the `window.postMessage` context.

I have removed one level of nesting in the `PortProvider#discover` that
make the method more readable.

I added some other suggestions from PR #3881.

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

This new method allows to unregister individual event listeners.

Bumps [eslint-plugin-jsx-a11y](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y) from 6.4.1 to 6.5.1.
- [Release notes](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/releases)
- [Changelog](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/master/CHANGELOG.md)
- [Commits](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/compare/v6.4.1...v6.5.1)

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

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

Sometimes there was some confusion between the `source` in the
`window#postMessage` context and the property in the message sent
through it. We believe it is better to replace the `SOURCE` by the
'hypothesis' string.

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

On Chrome, transferring the port more than once causes an error:

```
const {port1, port2} = new MessageChannel();
window.postMessage('hello', '*', [port1])
window.postMessage('hello', '*', [port1])
> Uncaught DOMException: Failed to execute 'postMessage' on 'Window': Port at index 0 is already neutered.
    at <anonymous>:1:8
```

As a result of `PortFinder` polling the host, `PortProvider` could
respond multiple times, specially when the polling frequency is high.

The combination of these two classes enable the discovery and
establishment of `MessageChannel` communication between pairs of frames.

It works with cross-origin frames.

It is the last step of this epic: https://github.com/hypothesis/client/issues/3533

Closes: https://github.com/hypothesis/client/issues/3695

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.1023.0 to 2.1028.0.
- [Release notes](https://github.com/aws/aws-sdk-js/releases)
- [Changelog](https://github.com/aws/aws-sdk-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/aws/aws-sdk-js/compare/v2.1023.0...v2.1028.0)

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Bumps [eslint-plugin-react-hooks](https://github.com/facebook/react/tree/HEAD/packages/eslint-plugin-react-hooks) from 4.2.0 to 4.3.0.
- [Release notes](https://github.com/facebook/react/releases)
- [Changelog](https://github.com/facebook/react/blob/main/packages/eslint-plugin-react-hooks/CHANGELOG.md)
- [Commits](https://github.com/facebook/react/commits/HEAD/packages/eslint-plugin-react-hooks)

---
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- dependency-name: eslint-plugin-react-hooks
  dependency-type: direct:development
  update-type: version-update:semver-minor
...
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Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 6.14.1 to 6.14.3.
- [Release notes](https://github.com/getsentry/sentry-javascript/releases)
- [Changelog](https://github.com/getsentry/sentry-javascript/blob/master/CHANGELOG.md)
- [Commits](https://github.com/getsentry/sentry-javascript/compare/6.14.1...6.14.3)

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  dependency-type: direct:development
  update-type: version-update:semver-patch
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Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.26.1 to 7.27.0.
- [Release notes](https://github.com/yannickcr/eslint-plugin-react/releases)
- [Changelog](https://github.com/yannickcr/eslint-plugin-react/blob/master/CHANGELOG.md)
- [Commits](https://github.com/yannickcr/eslint-plugin-react/compare/v7.26.1...v7.27.0)

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- dependency-name: eslint-plugin-react
  dependency-type: direct:development
  update-type: version-update:semver-minor
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Bumps [rollup](https://github.com/rollup/rollup) from 2.59.0 to 2.60.0.
- [Release notes](https://github.com/rollup/rollup/releases)
- [Changelog](https://github.com/rollup/rollup/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rollup/rollup/compare/v2.59.0...v2.60.0)

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  dependency-type: direct:development
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Bumps [enzyme-adapter-preact-pure](https://github.com/preactjs/enzyme-adapter-preact-pure) from 3.2.0 to 3.3.0.
- [Release notes](https://github.com/preactjs/enzyme-adapter-preact-pure/releases)
- [Changelog](https://github.com/preactjs/enzyme-adapter-preact-pure/blob/master/CHANGELOG.md)
- [Commits](https://github.com/preactjs/enzyme-adapter-preact-pure/compare/v3.2.0...v3.3.0)

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  dependency-type: direct:development
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There seems to be a pattern in our code that if a fake clock is used on
a single `it` function, the clock should be created and restored within
that function. I think it is beneficial from a readability perspective.
On the other hand, if the clock is used in several test functions it
should be declared on a parent context and restored on an `afterEach`
function.

I simplified an unrelated test using async.

On `destroy` the PDF integration clean up the `<hypothesis-banner>`
element, if any.

By default TypeScript includes all `@types/<name>` packages. This includes
@types/node, though we don't use it directly, because it is a transitive
dependency (see `yarn why @types/node`).

As a result Node's globals are added to the environment TS sees, even though
they don't really exist when our code runs in a browser. These globals
include overloads for `setTimeout` and `setInterval` which return a different
type (`Timeout`), causing spurious errors when assigning the result to a number.

Fix the problem by using the `types` option [1] in tsconfig.json to explicitly
specify which `@types/<name>` packages  to include when checking code in src/
This does mean that if we ever intentionally add @types packages which declare
globals (eg. for mocha), we'll need to explicitly list them here.

In the process the ES target was updated to fix an error about a `BigInt` reference.

[1] https://www.typescriptlang.org/tsconfig#types

`useRef(null as T|null)` now returns a `{ current: T|null }` instead of
`{ current: T }` as it did before. ie. it no longer drops the the null.
This makes sense but conflicted with a pattern we used in many places to
create a non-null ref: `useRef(/** @type {T|null} */ (null))`.

Resolve this by changing all non-nullable refs, for elements which are
set after the initial render, to cast the `useRef` result instead of the
init value.

```
const nonNullRef = /** @type {{ current: T }} */ (useRef());
```

Bumps [preact](https://github.com/preactjs/preact) from 10.5.13 to 10.5.15.
- [Release notes](https://github.com/preactjs/preact/releases)
- [Commits](https://github.com/preactjs/preact/compare/10.5.13...10.5.15)

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- @actions/core was used when the @hypothesis/frontend-shared package was
  part of the client repo.
- postcss-url was used prior to the introduction of
  @hypothesis/frontend-build