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The bookmarks bar appears right below the channel header on Professional and Enterprise licensed servers. The bookmarks bar is not visible on Team edition servers.
Mobile
The bookmarks bar should be hidden by default on mobile if there are no bookmarks.
If there are bookmarks however, the bookmark bar would appear.
There is no way to hide/show the bookmarks bar on a user+channel level on mobile.
The plus button for adding bookmarks is fixed to the right, while the bookmarks scroll horizontally.
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Desktop
The bookmark bar would appear on a channel by default even if you do not have a bookmark.
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