fix: content style (#6608)

Co-authored-by: 杨国璇 <ygx@192.168.1.8>

frontend/src/css/lib-content-view.css

@@ -9,7 +9,6 @@
   top: 48px;
   width: 100%;
   height: calc(100% - 48px);
-  border-top: 1px solid #e8e8e8;
   flex-direction: row;
   min-height: 0;
 }

frontend/src/pages/wikis/wikis.js

@@ -284,7 +284,7 @@ class Wikis extends Component {
                     <DropdownToggle tag="i" className="px-1">
                       <span className="sf3-font sf3-font-down" aria-hidden="true"></span>
                     </DropdownToggle>
-                    <DropdownMenu>
+                    <DropdownMenu positionFixed={true}>
                       <DropdownItem onClick={() => {this.toggelAddWikiDialog();}}>{gettext('Add Wiki')}</DropdownItem>
                     </DropdownMenu>
                   </Dropdown>