(1) Html設置
<mat-menu #settingNotificationMenu="matMenu" class="aa">
<div class="triangle"></div>
...
</mat-menu>
（2）CSS設置
::ng-deep.mat-menu-panel.aa {
...
.triangle {
border-left: 9px solid transparent;
border-right: 9px solid transparent;
border-bottom: 9px solid var(--color-primary-white);
position: absolute;
left: 495px;
margin-left: -5px;
opacity: 1;
margin-top: -15px;
}
}
顯示效果：
