This issue of UltraCompare for Windows v18.00 is fixed with v126.96.36.199. The command Switch Panes with its default hotkey assignment Alt+Ctrl+S works again and switches input focus between the recent two text file comparison panes.
The command switches active pane between left and right pane on a text comparison of two text files. The input focus is always set to recent active line in the current pane and active line is displayed highlighted in both file panes.
The behavior is the same on a 3-way text comparison which means it is not possible to switch input focus with Alt+Ctrl+S to third recent pane. So if input focus is set with pointing device first on middle pane, next on right pane and last on left pane, the command Switch Panes switches input focus respectively active pane between right and left pane. The middle pane cannot be activated with Alt+Ctrl+S. So with left pane being currently the active pane it is necessary to use the pointing device to make the middle pane the active pane. Then command Switch Panes switches active pane between left and middle pane and right pane must be activated with the pointing device.
I wrote an enhancement request for command Switch Panes to rotate between all three panes in recent active order on a 3-way comparison session.
Best regards from an UC/UE/UES for Windows user from Austria