On windows, Notepad++ compare plugin let's you compare unsaved files. So to compare two texts copied from elsewhere, just make two new tabs and paste the texts. Compare plugin will happily compare line by line.

On Linux I havent found something similar. The closes is Kate, but you still have to save tmp1.txt and tmp2.txt , and remove the clutter when finished.

Does anybody know a compare app that just lets you paste two text blocks without saving files first?