filter relative_url should keep absolute urls with scheme/authority (#6490)

Merge pull request 6490

lib/jekyll/filters/url_filters.rb

@@ -20,13 +20,16 @@ module Jekyll
         ).normalize.to_s
       end
 
-      # Produces a URL relative to the domain root based on site.baseurl.
+      # Produces a URL relative to the domain root based on site.baseurl
+      # unless it is already an absolute url with an authority (host).
       #
       # input - the URL to make relative to the domain root
       #
       # Returns a URL relative to the domain root as a String.
       def relative_url(input)
         return if input.nil?
+        return input if Addressable::URI.parse(input.to_s).absolute?
+
         parts = [sanitized_baseurl, input]
         Addressable::URI.parse(
           parts.compact.map { |part| ensure_leading_slash(part.to_s) }.join

test/test_filters.rb

@@ -526,6 +526,21 @@ class TestFilters < JekyllUnitTest
         filter = make_filter_mock({ "baseurl" => Value.new(proc { "/baseurl/" }) })
         assert_equal "/baseurl#{page_url}", filter.relative_url(page_url)
       end
+
+      should "transform protocol-relative url" do
+        url = "//example.com/"
+        assert_equal "/base//example.com/", @filter.relative_url(url)
+      end
+
+      should "not modify an absolute url with scheme" do
+        url = "file:///file.html"
+        assert_equal url, @filter.relative_url(url)
+      end
+
+      should "not normalize absolute international URLs" do
+        url = "https://example.com/错误"
+        assert_equal "https://example.com/错误", @filter.relative_url(url)
+      end
     end
 
     context "strip_index filter" do