fix(js): compute panel top position with getBoundingClientRect

[yyyanghj]

Close #535

The autocomplete container's parent element becomes offsetParent when there is a css declaration position:relative, where offsetTop is calculated relative to the parent element and not the actual position of the container in the page.

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 1e4e436:

Sandbox	Source
@algolia/autocomplete-example-github-repositories-custom-plugin	Configuration
@algolia/autocomplete-example-multiple-datasets-with-headers	Configuration
@algolia/autocomplete-example-playground	Configuration
@algolia/autocomplete-example-query-suggestion-with-categories	Configuration
@algolia/autocomplete-example-query-suggestions-with-hits	Configuration
@algolia/autocomplete-example-query-suggestions-with-inline-categories	Configuration
@algolia/autocomplete-example-query-suggestions-with-recent-searches-and-categories	Configuration
@algolia/autocomplete-example-query-suggestions-with-recent-searches	Configuration
@algolia/autocomplete-example-query-suggestions	Configuration
@algolia/autocomplete-example-react-renderer	Configuration
frosty-hill-pdleg	Issue #535

[francoischalifour]

Thank you @yyyang1996! I updated the tests in 1e4e436.