Tools > Options > Tabs
When working with multiple sessions simultaneously, Beyond Compare can open them in independent windows, or under tabs in a single window. These options control that behavior.
To control new sessions
Pick either A new window or A new tab for newly created sessions.
To control child sessions
Pick either A new window or A new tab for newly created file sessions that are opened directly from a folder session.
To control miscellaneous tab behavior
Set the various options to reflect your preferences.
If you unmark both the Closing last remaining tab closes the window and Hide tab bar if single tab checkboxes then closing the last tab will return to the Home view.