Currently, bitstreams open in the main window if the file size is below the threshold value. It would be better to open it in a new tab.
Users go back and forth. As the bitstream and metadatas may differ, it would be better to see both at the same time in different tabs.

Some discussion may be seen on mail thread: https://groups.google.com/g/dspace-community/c/6DzqJsv2mcI/m/lmKlvI-xBgAJ