Ability to click in the entry field of a drop-down menu

[j-boileau]

Reproduction link

https://ng-zorro-antd-start-s6vedu.stackblitz.io

Steps to reproduce

• Create a drop-down menu with search enabled
• Type in the search input
• Click on the inputed text to place the cursor

What is expected?

Have the option to click in the input box of the drop-down menu with the search option activated so as not to depend exclusively on the keyboard to modify the search.

What is actually happening?

The drop-down menu closes when clicking in the search box

Environment	Info
ng-zorro-antd	8.3.0
Browser	Google Chrome - Version 77.0.3865.90 (Build officiel) (64 bits)

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[vthinkxie]

Hi @j-boileau
it is caused by the same reason as #4259
there is a backdrop created by @angular/cdk to help detect the click outside event when open the dropdown, so there is no way to get text cursor worked when the dropdown opened now

[wenqi73]

closed via #5925.