There is no Difference Summary popup window because the compare process has completely changed. In previous versions UltraCompare did first the complete compare and you had to wait until the comparison finished. Then the difference summary report was shown which could be also opened later again. Now the comparison is done in a background thread and the UltraCompare window shows you the file contents or folder contents while comparison is still in progress. The background comparison thread writes the differences into the output window.
However, you don't need to have the output window permanently open. You can toggle the visibility of the output window via the menu item, a click on the toolbar symbol (most right one on the Merge toolbar) or by hotkey. You can enable auto hide mode of the output window to save space and it opens when you move the mouse pointer of the "tab" of the output window. And finally you can undock the output window by double clicking on the title bar of the window which results in a floating window and is practically the same as the popup window in the previous version.