Is there a diff tool that allows copy-paste

Try WinMerge. It'll do that.

Steps:

1. Download and install winmerge
2. Open WinMerge & Create new <CTRL+N>
3. Paste into left & right, then refresh <F5>

In case anyone comes here looking for a tool for Macs that can do this, it seems that there are two tools that can do just this.

• Beyond Compare, the Mac version is currently in beta.

• Kaleidoscope app

  1. Copy first text
  2. File -> New from Clipboard
  3. Copy second text
  4. Edit -> Paste to comparison

Unfortunately, it doesn't seem to be possible to change the texts once they've been pasted.

There is also a similar (closed) question (the question itself was for a Mac tool, but at least one answer has an alternative diff tool for Windows):

Diff tool for Mac without saving text to files

For those who use Atom, there’s the split-diff package.