Ng-zorro-antd: when tree-select has many leaf nodes, no scroll bar is displayed

[tangjwtj]

Reproduction link

https://stackblitz.com/edit/ng-zorro-antd-start-4clgvs

Steps to reproduce

1. Construct a two-layer tree
2. Tree has many leaf nodes

What is expected?

Show scroll bar

What is actually happening?

Don't show scroll bar, the leaf nodes not full displayed

| Environment | Info |
|---|---|
| ng-zorro-antd | 7.3.3 |
| Browser | chrome |

[hsuanxyz]

@tangjwt You can set the max-height in [nzDropdownStyle].

 <nz-tree-select [nzDropdownStyle]="{ 'max-height': '300px' }"></nz-tree-select>