Usually the folder-merge list view remains visible while a single file is compared or merged. With the mouse you can move the splitter bar that separates the file list from the text-diff windows. If you don't want this, you can disable Folder → Folder && Text Split Screen View menu item. Then you can use Folder → Toggle View menu item 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 Folder menu. You can also directly merge a single file by selecting it and choosing Merge → Merge Current File menu item. On saving the result, the status will be set to done, and the file will not be merged again if a folder merge is started.

But note that this status information will be lost when you rerun a folder scan: Folder → 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 Folder → Compare Explicitly Selected Files or Folder → Merge Explicitly Selected Files menu item. 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 folders a new window will be opened.