This is great. Thanks!

I have two observations:

1. There's still need for adding a new entry in the README documenting this new cusotm matcher.
2. At the moment, CI checks are not green. More about how to solve this one below.

About CI checks not passing: you need to add assertions to the tests that make it run the negative path. That is, expectations that when you call it in a way that it should fail, it does fail.

Here are some ideas.

// usage with .not
expect(element).not.toHaveSelection(
  'something that DOES NOT MATCH the current selection'
)

// same assertion without the .not should fail
expect(() => {
  expect(element).toHaveSelection(
    'something DOES NOT MATCH the current selection'
  )
}).toThrow()

// a valid assertion with the .not in place should fail
expect(() => {
  expect(element).not.toHaveSelection(
    'something that DOES MATCH the current selection'
  )
}).toThrow()

Try to sprinkle these in some test cases.

You can then run npm run validate and it will give you a coverage report. Unless it's at 100%, CI will not pass. If you keep stuck at not being able to get it to 100% let me know and push your work anyway. I may be able to help.