End to end test for branch switcher fetch policy

app/assets/javascripts/pipeline_editor/components/commit/commit_form.vue

@@ -103,6 +103,7 @@ export default {
           v-model="targetBranch"
           class="gl-font-monospace!"
           required
+          data-qa-selector="target_branch_field"
         />
         <gl-form-checkbox
           v-if="!isCurrentBranchTarget"

app/assets/javascripts/pipeline_editor/components/file_nav/branch_switcher.vue

@@ -211,6 +211,7 @@ export default {
     :header-text="$options.i18n.dropdownHeader"
     :text="currentBranch"
     icon="branch"
+    data-qa-selector="branch_selector_button"
   >
     <gl-search-box-by-type :debounce="$options.inputDebounce" @input="setSearchTerm" />
     <gl-dropdown-section-header>
@@ -228,6 +229,7 @@ export default {
           :key="branch"
           :is-checked="currentBranch === branch"
           :is-check-item="true"
+          data-qa-selector="menu_branch_button"
           @click="selectBranch(branch)"
         >
           {{ branch }}

qa/qa.rb

@@ -297,6 +297,10 @@ module QA
         autoload :New, 'qa/page/project/pipeline/new'
       end
 
+      module PipelineEditor
+        autoload :Show, 'qa/page/project/pipeline_editor/show'
+      end
+
       module Tag
         autoload :Index, 'qa/page/project/tag/index'
         autoload :New, 'qa/page/project/tag/new'

qa/qa/page/project/pipeline_editor/show.rb

+# frozen_string_literal: true
+
+module QA
+  module Page
+    module Project
+      module PipelineEditor
+        class Show < QA::Page::Base
+          view 'app/assets/javascripts/pipeline_editor/components/file_nav/branch_switcher.vue' do
+            element :branch_selector_button
+            element :menu_branch_button
+          end
+
+          view 'app/assets/javascripts/pipeline_editor/components/commit/commit_form.vue' do
+            element :target_branch_field
+          end
+
+          def has_branch_selector_button?
+            has_element? :branch_selector_button
+          end
+
+          def click_branch_selector_button
+            wait_until(reload: false) do
+              has_element?(:branch_selector_button)
+            end
+            click_element(:branch_selector_button, skip_finished_loading_check: true)
+          end
+
+          def select_branch_from_dropdown(branch_to_switch_to)
+            wait_until(reload: false) do
+              has_element?(:menu_branch_button)
+            end
+            click_element(:menu_branch_button, text: branch_to_switch_to, skip_finished_loading_check: true)
+          end
+
+          def target_branch_name
+            wait_until(reload: false) do
+              has_element?(:target_branch_field)
+            end
+            find_element(:target_branch_field, skip_finished_loading_check: true).value
+          end
+        end
+      end
+    end
+  end
+end

qa/qa/page/project/sub_menus/ci_cd.rb

@@ -20,6 +20,24 @@ module QA
               click_element(:sidebar_menu_link, menu_item: 'CI/CD')
             end
           end
+
+          def go_to_pipeline_editor
+            hover_ci_cd_pipelines do
+              within_submenu do
+                click_element(:sidebar_menu_item_link, menu_item: 'Editor')
+              end
+            end
+          end
+
+          private
+
+          def hover_ci_cd_pipelines
+            within_sidebar do
+              find_element(:sidebar_menu_link, menu_item: 'CI/CD').hover
+
+              yield
+            end
+          end
         end
       end
     end

qa/qa/specs/features/browser_ui/4_verify/pipeline/pipeline_editor_branch_switcher_spec.rb

+# frozen_string_literal: true
+
+module QA
+  RSpec.describe 'Verify' do
+    describe 'Pipeline editor', :requires_admin do
+      let(:project) do
+        Resource::Project.fabricate_via_api! do |project|
+          project.name = 'pipeline-editor-project'
+        end
+      end
+
+      let!(:commit) do
+        Resource::Repository::Commit.fabricate_via_api! do |commit|
+          commit.project = project
+          commit.commit_message = 'Add .gitlab-ci.yml'
+          commit.add_files(
+            [
+              {
+                file_path: '.gitlab-ci.yml',
+                content: default_file_content
+              }
+            ]
+          )
+        end
+      end
+
+      let!(:production_push) do
+        Resource::Repository::Push.fabricate! do |push|
+          push.repository_http_uri = project.repository_http_location.uri
+          push.branch_name = 'production'
+          push.file_name = '.gitlab-ci.yml'
+          push.file_content = production_file_content
+        end
+      end
+
+      let(:default_file_content) do
+        <<~YAML
+          stages:
+            - test
+          
+          initialize:
+            stage: test
+            script:
+              - echo "initialized in #{project.default_branch}"
+        YAML
+      end
+
+      let(:production_file_content) do
+        <<~YAML
+          stages:
+            - test
+          
+          initialize:
+            stage: test
+            script:
+              - echo "initialized in production"
+        YAML
+      end
+
+      before do
+        Runtime::Feature.enable(:pipeline_editor_branch_switcher)
+        Flow::Login.sign_in
+        project.visit!
+        Page::Project::Menu.perform(&:go_to_pipeline_editor)
+      end
+
+      after do
+        Runtime::Feature.disable(:pipeline_editor_branch_switcher)
+        project.remove_via_api!
+        Page::Main::Menu.perform(&:sign_out)
+      end
+
+      it 'can switch branches and target branch field updates accordingly', testcase: 'https://gitlab.com/gitlab-org/quality/testcases/-/issues/1856' do
+        Page::Project::PipelineEditor::Show.perform do |show|
+          expect(show).to have_branch_selector_button
+
+          show.click_branch_selector_button
+          show.select_branch_from_dropdown(production_push.branch_name)
+
+          expect(show.target_branch_name).to eq(production_push.branch_name)
+
+          show.click_branch_selector_button
+          show.select_branch_from_dropdown(project.default_branch)
+
+          expect(show.target_branch_name).to eq(project.default_branch)
+        end
+      end
+    end
+  end
+end