fix: optimize scroll position observer after video fullscreen exit

[ihor-romaniuk]

This merge request contains an optimization for toggling video xblock full-screen mode and saving the previous window top offset position on exit from the full-screen state.

With this refactor, we call scroll after exit full-screen mode only and not depends on xblock resize because sometimes the resize event comes before exiting full screen mode, and we don't need to call scroll every time on resize.

Related PR to quince branch: #1370

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.28%. Comparing base (72381a7) to head (79f0b6a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1371   +/-   ##
=======================================
  Coverage   88.28%   88.28%           
=======================================
  Files         292      292           
  Lines        5002     5002           
  Branches     1269     1269           
=======================================
  Hits         4416     4416           
  Misses        570      570           
  Partials       16       16           

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[itsjeyd]

Hey @ihor-romaniuk, thank you for this contribution! I marked it as ready for review, but let me know if you were planning to work on the changes some more.

[arbrandes]

Makes sense to me.

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@ihor-romaniuk 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.

[itsjeyd]

Hey @arbrandes, would you mind giving the Quince backport for this PR (#1370) a quick look, too?

[ihor-romaniuk]

@arbrandes and also take a look at Redwood backport PR #1405, please.