In the Windows version (other versions not tested), when you use "File" tab to computer a file hash (either by using Select file or drag&dropping it), the file handle is kept open after file was fully processed.
Processing it twice keeps 2 handles, etc.
This is problematic since it prevent some operations like renaming file in Explorer, or "gently" removing a USB key, etc. without first closing the program.
IIRC same happens when using "Compare Directories".
There shouldn't be any reason to keep a file handle open once it has been processed.