Mobile MR diffs overscroll problem

What does this MR do?

For #285016 (closed)

Adds a computation to the list of heights that determine the contentTop.

The "unresolved threads container" should be included in computations that wish to determine what the offset from the top of the browser is before content can begin. In mobile views, that container isn't included, which causes a scroll positioning problem.

This MR depends on another MR, and includes those changes as a matter of fact after a rebase.
The relevant commits in this MR are:

• Implementation and a small DRY refactor
• Changelog

Screenshots (strongly suggested)

Before (master after !47970 (merged) is merged)	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team