A call to `updateBoxSizes` was scheduled on every `resize` event after a
delay, instead of being debounced as intended. Fix this by using the
lodash debounce utility.

When reading the code top to bottom, the `expectedBoxOffsetAndSize`
function will make more sense after reading `createCharBoxes`.

This makes it easier to see what arguments are expected reading the
code.

Mapping each line in the text data in the test to a line in the
generated bounding boxes is more natural and also lets us test multiple
words on a single line.

Use fractions instead of percentages for the character and line metrics
in ImageTextLayer tests, for consistency with the ImageTextLayer API.

Support VitalSource books which are created from PDFs rather than EPUBs.
These books have the same frame structure as EPUBs, but use one HTML
page per PDF page in the inner frame, rather than one HTML page per
chapter as is more common for EPUBs.

VS PDFs do not contain a selectable text layer. Instead they consist of
a high-resolution rendered image of the current page plus data about the
content and location of text characters embedded in a script tag in the
page which sets a `window.innerPageData` variable.

To support this in Hypothesis, we generate a hidden text layer from the
`innerPageData` data and overlay it on top of the rendered image. This
hidden text layer is similar to what PDF.js creates for us in our
standard PDF viewer.

Implement a class that creates a transparent text layer on top of an
image containing text. This is useful for supporting annotation in
documents which are rendered images or canvases that don't have
browser-accessible text in them, such as VitalSource.

It is the caller's responsibility to determine the location and
character value of text in the image.

This class is used by elements created by placeholder.js in the host
page, not a shadow root.

With the exception of highlight styles, the styles in annotator.css are
now only used by UI elements contained within shadow roots. As a result,
we can stop loading annotator.css as a stylesheet into the host page,
preventing these styles from affecting the host page.

 - Stop loading annotator.css into the host page as a stylesheet.
   Instead add it as a preload link instead, ready for use by shadow
   roots.

 - Move highlights styles into a separate bundle and load that in the
   host page.

Fixes https://github.com/hypothesis/client/issues/2979

All of our target browsers now support shadow DOM [1]. Make it mandatory so it
is clear that all annotator UI elements can rely on Shadow DOM and thus don't
need to worry about their internal styles affecting or being affected by the
host page.

[1] https://caniuse.com/?search=shadow%20dom

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

There are two methods to get the host port from `PortProvider`:

- `#hostPortFrom` which returns the host port exclusively from the
  `sidebar-host` channel.

- `#on` allows to register a listener that is fired upon requests
  of host frames for channels other than the `sidebar-host`.

This is an attempt to simplify the API of `PortProvider` and merge both
methods into one.

Advantages:

- simplifies the `Sidebar` constructor signature (no need to pass the
  host port)

- avoids the pitfall of creating host-sidebar RPC channel before
  registering the RPC methods

- prepares for future guest-host channel see https://github.com/hypothesis/client/blob/ad93317debdac0d2c3abec83eb9d690934632ef3/src/annotator/index.js#L64-L66 (where the `#on` method is use to create the guest-host channel).

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

1. I converted `_setupSyncToGuest` and `_setupSyncFromGuests` into
   conventional class methods.

2. Added a new method, `_setupHostEvents` for sidebar-host
   communication.

3. Invoke these three methods above, from the constructor.

The `PortProvider._channels` map was used only to keep track of which
channel requests have been handled for a particular frame. The
`WeakMap<Window, MessageChannel>` was in fact used as a set rather than
a map. Only the presence of entries affected behavior, not the stored
MessageChannel values.

Change the data structure and comments to make its purpose clearer.

This comment provides general information about how MessagePorts work,
but isn't directly related to any of the surrounding code.

Of the two comments here, the second one is the most useful information
to consumers of the PortProvider and PortFinder interfaces, but it is
well covered by eg. MDN's documentation on MessagePort.

Rework the code to make the code path for the sidebar-host channel more
similar to other channels. As a result the `_sendPorts` helper was only
called in one place and can be inlined where it is called to make the
overall logic easier to follow.

Update the `MarkdownEditor` component to use `Icon`, `IconButton`,
`LabeledButton` and `Link` where relevant. Reduce complexity of
local component CSS and use utility styles where possible. Extract
button and link style overrides to `frontend_shared` SASS module.

Part of https://github.com/hypothesis/client/issues/3876
Part of https://github.com/hypothesis/frontend-shared/issues/232

Bumps [eslint](https://github.com/eslint/eslint) from 7.32.0 to 8.3.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/main/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v7.32.0...v8.3.0)

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  dependency-type: direct:development
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Bumps [rollup](https://github.com/rollup/rollup) from 2.60.0 to 2.60.1.
- [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/v2.60.0...v2.60.1)

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  dependency-type: direct:development
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Bumps [@babel/preset-env](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-env) from 7.16.0 to 7.16.4.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.16.4/packages/babel-preset-env)

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  dependency-type: direct:development
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Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.1028.0 to 2.1034.0.
- [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/v2.1028.0...v2.1034.0)

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- dependency-name: aws-sdk
  dependency-type: direct:development
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Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.27.0 to 7.27.1.
- [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/v7.27.0...v7.27.1)

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- dependency-name: eslint-plugin-react
  dependency-type: direct:development
  update-type: version-update:semver-patch
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Bumps [reselect](https://github.com/reduxjs/reselect) from 4.1.2 to 4.1.4.
- [Release notes](https://github.com/reduxjs/reselect/releases)
- [Changelog](https://github.com/reduxjs/reselect/blob/master/CHANGELOG.md)
- [Commits](https://github.com/reduxjs/reselect/compare/v4.1.2...v4.1.4)

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- dependency-name: reselect
  dependency-type: direct:development
  update-type: version-update:semver-patch
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Bumps [typescript](https://github.com/Microsoft/TypeScript) from 4.4.4 to 4.5.2.
- [Release notes](https://github.com/Microsoft/TypeScript/releases)
- [Commits](https://github.com/Microsoft/TypeScript/compare/v4.4.4...v4.5.2)

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  dependency-type: direct:development
  update-type: version-update:semver-minor
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Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 6.14.3 to 6.15.0.
- [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/6.14.3...6.15.0)

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  dependency-type: direct:development
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Bumps [karma](https://github.com/karma-runner/karma) from 6.3.8 to 6.3.9.
- [Release notes](https://github.com/karma-runner/karma/releases)
- [Changelog](https://github.com/karma-runner/karma/blob/master/CHANGELOG.md)
- [Commits](https://github.com/karma-runner/karma/compare/v6.3.8...v6.3.9)

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  dependency-type: direct:development
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Bumps [@hypothesis/frontend-shared](https://github.com/hypothesis/frontend-shared) from 4.1.0 to 4.3.0.
- [Release notes](https://github.com/hypothesis/frontend-shared/releases)
- [Changelog](https://github.com/hypothesis/frontend-shared/blob/main/CHANGELOG.md)
- [Commits](https://github.com/hypothesis/frontend-shared/compare/v4.1.0...v4.3.0)

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  dependency-type: direct:development
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The current version of `isSourceWindow` raised an error for (1) older
browser that doesn't support ServiceWorker and (2) web pages served from
non-secured http connections.

I tested this alternative fix but it didn't work for me in the latest
version of Chrome:
https://stackoverflow.com/questions/6229301/a-clean-way-of-checking-whether-an-object-is-an-instance-of-window-constructor/6229603#6229603

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.

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.