on long strings, rspec may not display the part of the string that does not match

I'm trying to track down an error in a flaky test. Unfortunately the string output presented by rspec does not display the problem.

The following test should be enough to demonstrate the problem:

describe 'a long string' do
  it 'shows the right part of the diff' do
    got = "a"*100 + "Z" + "a"*100
    want = "a" * (1 + 100*2)
    expect(got).to eq(want)
  end
end

Yields the following output:

Failures:

  1) a long string shows the right part of the diff
     Failure/Error: expect(got).to eq(want)

       expected: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa...aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
            got: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa...aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"

       (compared using ==)
     # ./longstring_spec.rb:5:in `block (2 levels) in <top (required)>'

The strings displayed by rspec are identical; the diff is in the truncated part. If the string is any shorter, the diff appears correctly.

Running on a Mac in iTerm2.

$ rspec --version
RSpec 3.7
  - rspec-core 3.7.1
  - rspec-expectations 3.7.0
  - rspec-mocks 3.7.0
  - rspec-support 3.7.1

Start by running the reproduction in longstring_spec.rb with the reported RSpec 3.7 versions and inspect the failure output for the differing character. Trace how RSpec formats and truncates expected and actual strings. Done means long-string failures expose the differing section instead of showing identical truncated values.