Usually the directory merge list view remains visible while a single file is compared or merged. With the mouse you can move the the splitter bar that separates the file list from the text-diff windows. If you don't want this, you can disable "Split Screen View" in the "Directory"-menu. Then you can use "Toggle View" in the "Directory"-menu to switch between the file list and the text-diff view that then occupy the full screen.
Probably you will prefer a simple double mouse click on a file in order to compare it. Nevertheless there also exists an entry in the "Directory"-menu. You can also directly merge a single file by selecting it and choosing "Merge current file" in the "Merge"-Menu. On saving the result, the status will be set to done, and the file will not be merged again if a directory merge is started.
But note that this status information will be lost when you rerun a directory scan: "Directory"-menu: "Rescan"
Sometimes you need to compare or merge files with different names (e.g. the current file and the backup in the same folder).
Select the exact file by clicking onto the icon in the column A, B or C. The first file selected thus will be marked with an "A", the second and third with "B" and "C" regardless on what column they are in. Only up to three files can be chosen like this.
Proceed by choosing "Compare Explicitely Selected Files" or "Merge Explicitely Selected Files" from the "Directory"-menu. For your convenience these menu entries also appear as context menu when you right-click the last selected file.
The comparison or merge of a file will happen in the same window. If this method is used for directories a new window will be opened.