In UltraCompare Professional this additional window can be toggled via Show Active Line Window in menu View. This window displays on a comparison of 2 text files the active line of both files. And with UC Prof. it is additionally possible to edit each line after double clicking on a line in this window.
But Show Active Line Window is missing in menu View in UC Lite v8.00.0.1027 as installed with UE v21.00. I do not know the reason.
However, it is possible to get this window displayed also in UC Lite v8.00.0.1027 by toggling the setting in registry. It is not possible in UC Lite to edit the active lines, but displaying them in the additional window is possible.
You need to run only once the command
reg.exe ADD "HKCU\Software\IDM Computer Solutions\UltraCompare Lite\Settings" /v ShowAdditionalFrames /t REG_DWORD /f /d 1
either in a command prompt window or from within UltraEdit via Advanced - DOS Command.
It is not necessary to have administrator privileges to execute this command as it changes only a double word value in registry of current user.
The value 1 enables the Active Line Window for next start of UC Lite and all further starts. So as long as the command is not executed once again with value 0 at the end, or closed from within UC Lite, the active line window is always displayed in UC Lite.
Please note that executing the command while UC Lite is already running has no effect. The setting is read from registry only on start of UC Lite and written back on exit.