README.md

To make SameSite=None cookies work in a rails application (or any rack-based ruby app) you need to also FILTER OUT any clients that don't work nicely with this new flag, namely clients who mistreat the None value or drop the cookie when incompatible values are seen.

application_controller.rb

class ApplicationControler < ActionController::Base
  
  ...
    
  def set_third_party_cookie(name, value, expires_in: 1.year)
    cookies.encrypted[name] = { :value => value.to_s, :expires_in => expires_in, :domain => :all, :same_site => SameSite.value(request.headers['User-Agent']), :secure => true, :httponly => true }.compact
  end
  
  ...

end

Gemfile

...
##
# Until this commit is merged and released by rack
#
gem 'rack', git: 'https://github.com/rack/rack.git', ref: 'c859bbf7b53cb59df1837612a8c330dfb4147392'
...

same_site.rb

class SameSite
  def self.value(agent)
    send_same_site_none?(agent) ? :none : nil
  end

  private

  ##
  # Based on https://www.chromium.org/updates/same-site/incompatible-clients
  #
  #  Don’t send `SameSite=None` to known incompatible clients.
  # 
  def self.send_same_site_none?(useragent)
    return true if useragent.blank?

    !same_site_none_incompatible?(useragent)
  end

  #  Classes of browsers known to be incompatible.
  # 
  def self.same_site_none_incompatible?(useragent)
    web_kit_same_site_bug?(useragent) ||
      drops_unrecognized_cookies?(useragent)
  end

  def self.web_kit_same_site_bug?(useragent)
    ios_version?(useragent, major: 12) ||
      (macosx_version?(useragent, major: 10, minor: 14) &&
        (safari?(useragent) || mac_embedded_browser?(useragent)))
  end

  def self.drops_unrecognized_cookies?(useragent)
    return !uc_browser_version_at_least?(useragent, major: 12, minor: 13, build: 2) if uc_browser?(useragent)

    chromium_based?(useragent) &&
      chromium_version_at_least?(useragent, major: 51) &&
      !chromium_version_at_least?(useragent, major: 67)
  end

  def self.ios_version?(useragent, major: nil)
    /\(iP.+; CPU .*OS (\d+)[_\d]*.*\) AppleWebKit\// =~ useragent

    $1 == major.to_s
  end

  def self.macosx_version?(useragent, major:nil, minor:nil)
    /\(Macintosh;.*Mac OS X (\d+)_(\d+)[_\d]*.*\) AppleWebKit\// =~ useragent

    $1 == major.to_s && $2 == minor.to_s
  end

  def self.safari?(useragent)
    /Version\/.* Safari\// === useragent && !chromium_based?(useragent)
  end

  def self.mac_embedded_browser?(useragent)
    /^(Mozilla\/[.\d]+ \(Macintosh;.*Mac OS X [_\d]+\) )|AppleWebKit\/[.\d]+ \(KHTML, like Gecko\)$/ === useragent
  end

  def self.chromium_based?(useragent)
    /Chrom(e|ium)/ === useragent
  end

  def self.chromium_version_at_least?(useragent, major: nil)
    /Chrom[^ \/]+\/(\d+)[.\d]* / =~ useragent

    $1.to_i >= major
  end

  def self.uc_browser?(useragent)
    /UCBrowser\// =~ useragent
  end

  def self.uc_browser_version_at_least?(useragent, major: nil, minor: nil, build: nil)
    /UCBrowser\/(\d+)\.(\d+)\.(\d+)[.\d]* / =~ useragent

    major_version = $1.to_i
    minor_version = $2.to_i

    if major_version != major
      return major_version > major
    elsif minor_version != minor
      return minor_version > minor
    end

    $3.to_i >= build
  end
end

In your mac_embedded_browser? method there's a pipe character (|) that should be a space ( ) based on the original incompatible clients code.

/^(Mozilla\/[.\d]+ \(Macintosh;.*Mac OS X [_\d]+\) ) AppleWebKit\/[.\d]+ \(KHTML, like Gecko\)$/ === useragent

I agree, the pipe should not be there. But there should also only be 1 space between the fist part and the second, so one of the spaces needs to be removed as well. I've tested with the 547 most common browser from this site and tried to manually verify all the incompatible cases.

I use this version (first parenthesis are not required, but makes it clearer)

/^(Mozilla\/[.\d]+ \(Macintosh;.*Mac OS X [_\d]+\)) AppleWebKit\/[.\d]+ \(KHTML, like Gecko\)$/ === useragent

These should all match, they End with (KHTML, like Gecko)

"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/605.1.15 (KHTML, like Gecko)"
https://developers.whatismybrowser.com/useragents/parse/1273795-webkit-based-browser-macos-webkit

"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_4) AppleWebKit/605.1.15 (KHTML, like Gecko)"
https://developers.whatismybrowser.com/useragents/parse/1253253-webkit-based-browser-macos-webkit

"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_3) AppleWebKit/605.1.15 (KHTML, like Gecko)"
https://developers.whatismybrowser.com/useragents/parse/1095784-webkit-based-browser-macos-webkit

This should not match, there's additional stuff at the end

"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36"
https://developers.whatismybrowser.com/useragents/parse/1302439-chrome-macos-blink"

This is why you use real user agent parser.