add set_cookie helper

spec/features/boards/boards_spec.rb

 require 'rails_helper'
 
 describe 'Issue Boards', js: true do
+  include CookieHelper
   include DragTo
 
   let(:group) { create(:group, :nested) }
@@ -13,7 +14,7 @@ describe 'Issue Boards', js: true do
     project.team << [user, :master]
     project.team << [user2, :master]
 
-    page.driver.set_cookie('sidebar_collapsed', 'true')
+    set_cookie('sidebar_collapsed', 'true')
 
     sign_in(user)
   end

spec/features/issues/filtered_search/visual_tokens_spec.rb

 require 'rails_helper'
 
 describe 'Visual tokens', js: true do
+  include CookieHelper
   include FilteredSearchHelpers
   include WaitForRequests
 
@@ -28,7 +29,7 @@ describe 'Visual tokens', js: true do
     sign_in(user)
     create(:issue, project: project)
 
-    page.driver.set_cookie('sidebar_collapsed', 'true')
+    set_cookie('sidebar_collapsed', 'true')
 
     visit project_issues_path(project)
   end

spec/features/merge_requests/diff_notes_avatars_spec.rb

 require 'spec_helper'
 
 feature 'Diff note avatars', js: true do
+  include CookieHelper
   include NoteInteractionHelpers
 
   let(:user)          { create(:user) }
@@ -22,7 +23,7 @@ feature 'Diff note avatars', js: true do
     project.team << [user, :master]
     sign_in user
 
-    page.driver.set_cookie('sidebar_collapsed', 'true')
+    set_cookie('sidebar_collapsed', 'true')
   end
 
   context 'discussion tab' do

spec/features/merge_requests/user_posts_diff_notes_spec.rb

 require 'spec_helper'
 
 feature 'Merge requests > User posts diff notes', :js do
+  include CookieHelper
+
   let(:user) { create(:user) }
   let(:merge_request) { create(:merge_request) }
   let(:project) { merge_request.source_project }
 
   before do
-    visit '/'
-    page.driver.browser.manage.add_cookie(name: 'sidebar_collapsed', value: 'true')
+    set_cookie('sidebar_collapsed', 'true')
 
     project.add_developer(user)
     sign_in(user)

spec/support/cookie_helper.rb

+# Helper for setting cookies in Selenium/WebDriver
+#
+module CookieHelper
+  def set_cookie(name, value, options = {})
+    # Selenium driver will not set cookies for a given domain when the browser is at `about:blank`.
+    # It also doesn't appear to allow overriding the cookie path. loading `/` is the most inclusive.
+    visit options.fetch(:path, '/') unless on_a_page?
+    page.driver.browser.manage.add_cookie(name: name, value: value, **options)
+  end
+
+  private
+
+  def on_a_page?
+    current_url = Capybara.current_session.driver.browser.current_url
+    current_url && current_url != '' && current_url != 'about:blank' && current_url != 'data:,'
+  end
+end