The demonstrated behavior is expected when the ImageManager is loaded under a small resolution, or inside a parent having smaller height than it does. There is a default min height needed for the dialog in order to be able to show its whole content (FileExplorer, preview area, buttons) and when the Dialog window get resized to a smaller height, the scroll-bars of its content get shown.
In case you are experiencing this behavior regardless of the browser viewport or the Editor's parent document height, please, send us the exact configuration needed for replication the isseu and we will examine it further.
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