Total Commander (Windows) and some of its many homages (for example, Krusader on Linux) have a "compare by content" selection under the "Files" menu item. This package is a split-window file explorer made for fast navigation/copying/other file operations between one place and another.
The "Compare by content" item will do a line-by-line text diff for text-type files, or show binary differences for anything else.
You would select a file from each of the windows to effect the comparison.
It's particularly effective if you are comparing two directories (using "Synchronize Dirs"), and find that one of the files is 'different'; you can find out what changed.