Add `base_url` to Static Site Editor payload

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/213282

Base_url is required for the Vue router.

app/views/projects/static_site_editor/show.html.haml

--# TODO: Remove after base URL is included in the model !30735
-- data = @config.payload.merge({ base_url: namespace_project_show_sse_path })
-
-#static-site-editor{ data: data }
+#static-site-editor{ data: @config.payload }

lib/gitlab/static_site_editor/config.rb

@@ -22,7 +22,8 @@ module Gitlab
           project: project.path,
           namespace: project.namespace.path,
           return_url: return_url,
-          is_supported_content: supported_content?.to_s
+          is_supported_content: supported_content?.to_s,
+          base_url: Gitlab::Routing.url_helpers.project_show_sse_path(project, full_path)
         }
       end
 
@@ -47,6 +48,10 @@ module Gitlab
       def file_exists?
         commit_id.present? && repository.blob_at(commit_id, file_path).present?
       end
+
+      def full_path
+        "#{ref}/#{file_path}"
+      end
     end
   end
 end

spec/lib/gitlab/static_site_editor/config_spec.rb

@@ -5,9 +5,10 @@ require 'spec_helper'
 describe Gitlab::StaticSiteEditor::Config do
   subject(:config) { described_class.new(repository, ref, file_path, return_url) }
 
-  let(:project) { create(:project, :public, :repository, name: 'project', namespace: namespace) }
-  let(:namespace) { create(:namespace, name: 'namespace') }
-  let(:repository) { project.repository }
+  let_it_be(:namespace) { create(:namespace, name: 'namespace') }
+  let_it_be(:project) { create(:project, :public, :repository, name: 'project', namespace: namespace) }
+  let_it_be(:repository) { project.repository }
+
   let(:ref) { 'master' }
   let(:file_path) { 'README.md' }
   let(:return_url) { 'http://example.com' }
@@ -24,10 +25,17 @@ describe Gitlab::StaticSiteEditor::Config do
         project: 'project',
         project_id: project.id,
         return_url: 'http://example.com',
-        is_supported_content: 'true'
+        is_supported_content: 'true',
+        base_url: '/namespace/project/-/sse/master%2FREADME.md'
       )
     end
 
+    context 'when file path is nested' do
+      let(:file_path) { 'lib/README.md' }
+
+      it { is_expected.to include(base_url: '/namespace/project/-/sse/master%2Flib%2FREADME.md') }
+    end
+
     context 'when branch is not master' do
       let(:ref) { 'my-branch' }
 
@@ -53,7 +61,7 @@ describe Gitlab::StaticSiteEditor::Config do
     end
 
     context 'when repository is empty' do
-      let(:project) { create(:project_empty_repo) }
+      let(:repository) { create(:project_empty_repo).repository }
 
       it { is_expected.to include(is_supported_content: 'false') }
     end