Print Bundler progress to stderr when invoking CLI directly

lib/ruby_lsp/setup_bundler.rb

@@ -16,7 +16,26 @@
 # the Ruby LSP without including the gem in their application's Gemfile while at the same time giving us access to the
 # exact locked versions of dependencies.
 
-Bundler.ui.level = :silent
+# Language servers can't print to stdout or that breaks the connection with the editor, but we still want to be able to
+# display progress coming from Bundler. Here we patch the basic Thor shell to always print to stderr, so that progress
+# appears in the editor without breaking the communication
+module Bundler
+  class Thor
+    module Shell
+      class Basic
+        module Patch
+          def stdout
+            $stderr
+          end
+        end
+
+        prepend(Patch)
+      end
+    end
+  end
+end
+
+Bundler.ui.level = :info
 
 module RubyLsp
   class SetupBundler

test/setup_bundler_test.rb

@@ -714,6 +714,32 @@ def test_invoke_cli_calls_bundler_directly_for_update
     end
   end
 
+  def test_progress_is_printed_to_stderr
+    Dir.mktmpdir do |dir|
+      Dir.chdir(dir) do
+        File.write(File.join(dir, "Gemfile"), <<~GEMFILE)
+          source "https://rubygems.org"
+          gem "rdoc"
+        GEMFILE
+
+        Bundler.with_unbundled_env do
+          capture_subprocess_io do
+            # Run bundle install to generate the lockfile
+            system("bundle install")
+          end
+
+          stdout, stderr = capture_subprocess_io do
+            RubyLsp::SetupBundler.new(dir, launcher: true).setup!
+          end
+
+          assert_match(/Bundle complete! [\d]+ Gemfile dependencies, [\d]+ gems now installed/, stderr)
+          assert_match(/Use `bundle info \[gemname\]` to see where a bundled gem is installed/, stderr)
+          assert_empty(stdout)
+        end
+      end
+    end
+  end
+
   private
 
   def with_default_external_encoding(encoding, &block)