fix(module:date-picker): overlay panel flicker when position changes

[wenqi73]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[codecov]

Codecov Report

Merging #4315 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4315      +/-   ##
==========================================
+ Coverage   92.15%   92.16%   +0.01%     
==========================================
  Files         520      520              
  Lines       11016    11020       +4     
  Branches     1997     1999       +2     
==========================================
+ Hits        10152    10157       +5     
  Misses        431      431              
+ Partials      433      432       -1

Impacted Files	Coverage Δ
...te-picker/lib/calendar/calendar-input.component.ts	100% <100%> (ø)	⬆️
components/date-picker/picker.component.ts	97.36% <100%> (+0.14%)	⬆️
components/time-picker/time-holder.ts	96.49% <0%> (+0.87%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f203afd...c37db90. Read the comment docs.

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit c37db90

https://deploy-preview-4315--ng-zorro-master.netlify.com