Currently, there is a single preview window. If a preview window is open, the user navigates to a different contact sheet and opens the preview window from that contact sheet, the new Preview window replaces the old one.
Request: add the option for each Contact sheet to have its own preview window. Double clicking on the image from the second contact sheet opens a second preview window. This allows users (especially on Windows which has native window docking) to compare images from two different folders easily without having to open up as a single contact sheet (which is not always ideal).