Closes #31: Add a check if page is scrolled down.

[Miraeld]

Description

Fixes #31
This PR adds a check to ensure that the beacon is not fired if the page is scrolled down before it runs.

Type of change

• Bug fix (non-breaking change which fixes an issue).

Detailed scenario

• Use - https://gist.github.com/DahmaniAdame/bd58d4be051f5995aa2318d341eb342a
• Scroll to the last image
• Hit refresh while making sure the beacon runs
• See the LCP image will be related to the current viewport

Technical description

Documentation

The _isPageScrolled() method is used in BeaconManager.js to check if the page is scrolled and bails out if it returns true. In-build test have been added as well.

New dependencies

None

Risks

None

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.
• Output messages (errors, notices, logs) are explicit enough for users to understand the issue and are actionnable.

Additional Checks

• In the case of complex code, I wrote comments to explain it.
• When possible, I prepared ways to observe the implemented system (logs, data, etc.).
• I added error handling logic when using functions that could throw errors (HTTP/API request, filesystem, etc.)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +3.19% (target: -1.00%)	✅ 100.00% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (78ca211)	578	349	60.38%
Head commit (4f760de)	645 (+67)	410 (+61)	63.57% (+3.19%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#32)	10	10	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[wordpressfan]

I think @Miraeld is not working today, I'll go ahead and do that small change by myself.