1 files changed, 24 insertions, 1 deletions
diff --git a/actionpack/lib/action_dispatch/routing/routes_proxy.rb b/actionpack/lib/action_dispatch/routing/routes_proxy.rb
index 7a6c2e95d8..587a72729c 100644
--- a/actionpack/lib/action_dispatch/routing/routes_proxy.rb
+++ b/actionpack/lib/action_dispatch/routing/routes_proxy.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "active_support/core_ext/array/extract_options"
 
 module ActionDispatch
@@ -31,7 +33,14 @@ module ActionDispatch
             def #{method}(*args)
               options = args.extract_options!
               options = url_options.merge((options || {}).symbolize_keys)
-              options.reverse_merge!(script_name: @script_namer.call(options)) if @script_namer
+
+              if @script_namer
+                options[:script_name] = merge_script_names(
+                  options[:script_name],
+                  @script_namer.call(options)
+                )
+              end
+
               args << options
               @helpers.#{method}(*args)
             end
@@ -41,6 +50,20 @@ module ActionDispatch
           super
         end
       end
+
+      # Keeps the part of the script name provided by the global
+      # context via ENV["SCRIPT_NAME"], which `mount` doesn't know
+      # about since it depends on the specific request, but use our
+      # script name resolver for the mount point dependent part.
+      def merge_script_names(previous_script_name, new_script_name)
+        return new_script_name unless previous_script_name
+
+        resolved_parts = new_script_name.count("/")
+        previous_parts = previous_script_name.count("/")
+        context_parts = previous_parts - resolved_parts + 1
+
+        (previous_script_name.split("/").slice(0, context_parts).join("/")) + new_script_name
+      end
     end
   end
 end