Linux

meld allows you to start an empty comparison and to paste content into it without saving.

diff(1)

Edit: oh I'm not sure you can paste two things into the terminal like that. Maybe kdiff3. If I'm really lazy I use this online one: https://editor.mergely.com/

diff <<EOF  
pastestuffhere  
EOF  

while this is running, paste the other stuff into terminal. I'm assuming diff reads stdin, if not given a filename.

https://meldmerge.org/

You could always run NP++ via WINE if you can't find a native Linux solution that you like.

vim's :diffthis

I quickly wrote a script that uses kdialog from KDE to input text in a box, then writes both files to a temporary file, compares with diff and outputs the difference in a text box again. At the end it deletes the temporary files (if they are not deleted, they will be removed automatically with next system boot anyway). It's a quick and dirty script.

I call it diffuse, for diff + use.

#!/usr/bin/env bash

title="Diff - Compare 2 Texts"
output_size="720x720"

file1="$(mktemp)"
file2="$(mktemp)"
file3="$(mktemp)"

kdialog --title "${title} (input)" --textinputbox "Input Text 1" >> "${file1}"
kdialog --title "${title} (input)" --textinputbox "Input Text 2" >> "${file2}"
diff -- "${file1}" "${file2}" >> "${file3}"
kdialog --title "${title} (diff)" --geometry "${output_size}" --textbox "${file3}"

rm -- "${file1}"
rm -- "${file2}"
rm -- "${file3}"

Edit: Forgot to mention the name of the script. Edit2: Totally wrong shebang line corrected.

For CLI, diff has already been mentioned; for a GUI application I'd recommend meld:

https://meldmerge.org/

It’s not a very Linux-y answer, but VSC does allow you to compare 2 pages for differences. Those pages can be unsaved or saved files.