The 'Browse by category box' shown at the top of the page is called the category quick filter. It is available in all reviewing modes, including galleries. The category quick filter allows reviewers to focus on a single category at a time. While it is enabled by default, this filter can be hidden if not required.
Hide the category quick filter
In the Manage workspace, go to Reviewing > Settings > Score sets
Select the score set you want to update
Open the 'Display' tab
Under 'Application list', deselect Category quick filter
Click Save
Good to know
Hiding the category quick filter also hides it from the reviewing leaderboard.
Grant managers will still have access to Advanced search filters.
