角材料展开面板导航栏问题

Question

我正在尝试为我的有角项目创建侧面导航栏。我希望导航栏支持可扩展项，因此我为此使用了扩展面板。问题是，当我展开导航项目时，标题移到上方的项目并与之重叠，并且可展开面板的内容位于下面的数据之上。下面，我附上我的一些代码和问题的屏幕截图。

谢谢

HTML                        

  process = Runtime.getRuntime().exec("systemctl restart httpd");
  BufferedReader br = new BufferedReader(new InputStreamReader(process.getInputStream()));
  while ((result = br.readLine()) != null)
    out = out + result + "\n";
  process.waitFor();
  System.out.println("exit: " + process.exitValue());
  process.destroy();
  return out;

SCSS

        <mat-expansion-panel-header class="expansion-header" [collapsedHeight]="'48px'" [expandedHeight]="'48px'">
          <mat-panel-title fxLayout="row" fxLayoutAlign="start center">
            <mat-icon>{{menuitem.icon}}</mat-icon>
            <span>{{menuitem.name}}</span>
          </mat-panel-title>
        </mat-expansion-panel-header>
        <mat-nav-list>
          <a class="expanded-nav-item" routerLinkActive="active-route" mat-list-item
            *ngFor="let submenuitem of menuitem.state" [routerLink]="submenuitem.state">
            <span>{{submenuitem.name}}</span>
          </a>
        </mat-nav-list>
      </mat-expansion-panel>
    </mat-accordion>