Consider viewport on element distance calculation

[wordpressfan]

Description

Part of wp-media/wp-rocket#6933

Here we will calculate the threshold below the viewport.

Type of change

• Bug fix (non-breaking change which fixes an issue).
• Enhancement (non-breaking change which improves an existing functionality).

Detailed scenario

Now we will calculate the threshold starting from the end of viewport as discussed here:

wp-media/wp-rocket#6929 (comment)

Technical description

Documentation

As mentioned in the issue itself we will consider having viewport height in the calculation.

New dependencies

No

Risks

One risk is mentioned and shared with product team here:
wp-media/wp-rocket#6933 (comment)
and we are fine with that.

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
✅ +0.60% (target: -1.00%)	✅ 76.92% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (96cb6c5)	571	287	50.26%
Head commit (ecdac7a)	578 (+7)	294 (+7)	50.87% (+0.60%)

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 (#28)	13	10	76.92%

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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