Fix links to subdirectories of a directory with a plus character in its path

app/helpers/tree_helper.rb

@@ -123,7 +123,7 @@ module TreeHelper
 
   # returns the relative path of the first subdir that doesn't have only one directory descendant
   def flatten_tree(root_path, tree)
-    return tree.flat_path.sub(%r{\A#{root_path}/}, '') if tree.flat_path.present?
+    return tree.flat_path.sub(%r{\A#{Regexp.escape(root_path)}/}, '') if tree.flat_path.present?
 
     subtree = Gitlab::Git::Tree.where(@repository, @commit.id, tree.path)
     if subtree.count == 1 && subtree.first.dir?

changelogs/unreleased/dm-flatten-tree-plus-chars.yml

@@ -0,0 +1,5 @@
+---
+title: Fix links to subdirectories of a directory with a plus character in its path
+merge_request:
+author:
+type: fixed

spec/helpers/tree_helper_spec.rb

@@ -8,6 +8,7 @@ describe TreeHelper do
   describe '.render_tree' do
     before do
       @id = sha
+      @path = ""
       @project = project
       @lfs_blob_ids = []
     end
@@ -61,6 +62,15 @@ describe TreeHelper do
         end
       end
     end
+
+    context 'when the root path contains a plus character' do
+      let(:root_path) { 'gtk/C++' }
+      let(:tree_item) { double(flat_path: 'gtk/C++/glade') }
+
+      it 'returns the flattened path' do
+        expect(subject).to eq('glade')
+      end
+    end
   end
 
   describe '#commit_in_single_accessible_branch' do