Add latest changes from gitlab-org/gitlab@master

2 files changed, 330 insertions, 429 deletions

diff --git a/doc/development/testing_guide/frontend_testing.md b/doc/development/testing_guide/frontend_testing.md
index 626d4147e6a..9dc06a4b8b8 100644
--- a/doc/development/testing_guide/frontend_testing.md
+++ b/doc/development/testing_guide/frontend_testing.md
@@ -164,7 +164,7 @@ Testing the `hasMetricTypes` computed prop would seem like a given, but to test
 
 Keep an eye out for these kinds of tests, as they just make updating logic more fragile and tedious than it needs to be. This is also true for other libraries.
 
-Some more examples can be found further down [below](#unit-testing-guidelines)
+Some more examples can be found in the [Frontend unit tests section](testing_levels.md#frontend-unit-tests)
 
 ### Don't test your mock
 
@@ -749,433 +749,22 @@ end
 
 ## Overview of Frontend Testing Levels
 
+Main information on frontend testing levels can be found in the [Testing Levels page](testing_levels.md).
+
 Tests relevant for frontend development can be found at the following places:
 
-- `spec/javascripts/` which are run by Karma (command: `yarn karma`) and contain
-  - [frontend unit tests](#frontend-unit-tests)
-  - [frontend component tests](#frontend-component-tests)
-  - [frontend integration tests](#frontend-integration-tests)
-- `spec/frontend/` which are run by Jest (command: `yarn jest`) and contain
-  - [frontend unit tests](#frontend-unit-tests)
-  - [frontend component tests](#frontend-component-tests)
-  - [frontend integration tests](#frontend-integration-tests)
-- `spec/features/` which are run by RSpec and contain
-  - [feature tests](#feature-tests)
+- `spec/javascripts/`, for Karma tests
+- `spec/frontend/`, for Jest tests
+- `spec/features/`, for RSpec tests
+
+RSpec runs complete [feature tests](testing_levels.md#frontend-feature-tests), while the Jest and Karma directories contain [frontend unit tests](testing_levels.md#frontend-unit-tests), [frontend component tests](testing_levels.md#frontend-component-tests), and [frontend integration tests](testing_levels.md#frontend-integration-tests).
 
 All tests in `spec/javascripts/` will eventually be migrated to `spec/frontend/` (see also [#52483](https://gitlab.com/gitlab-org/gitlab-foss/issues/52483)).
 
-In addition, there used to be feature tests in `features/`, run by Spinach.
-These were removed from the codebase in May 2018 ([#23036](https://gitlab.com/gitlab-org/gitlab-foss/issues/23036)).
+Before May 2018, `features/` also contained feature tests run by Spinach. These tests were removed from the codebase in May 2018 ([#23036](https://gitlab.com/gitlab-org/gitlab-foss/issues/23036)).
 
 See also [Notes on testing Vue components](../fe_guide/vue.html#testing-vue-components).
 
-### Frontend unit tests
-
-Unit tests are on the lowest abstraction level and typically test functionality that is not directly perceivable by a user.
-
-```mermaid
-graph RL
-    plain[Plain JavaScript];
-    Vue[Vue Components];
-    feature-flags[Feature Flags];
-    license-checks[License Checks];
-
-    plain---Vuex;
-    plain---GraphQL;
-    Vue---plain;
-    Vue---Vuex;
-    Vue---GraphQL;
-    browser---plain;
-    browser---Vue;
-    plain---backend;
-    Vuex---backend;
-    GraphQL---backend;
-    Vue---backend;
-    backend---database;
-    backend---feature-flags;
-    backend---license-checks;
-
-    class plain tested;
-    class Vuex tested;
-
-    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
-    classDef label stroke-width:0;
-    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
-
-    subgraph " "
-    tested;
-    mocked;
-    class tested tested;
-    end
-```
-
-<div id="unit-testing-guidelines"></div>
-#### When to use unit tests
-
-<details>
-  <summary>exported functions and classes</summary>
-  Anything that is exported can be reused at various places in a way you have no control over.
-  Therefore it is necessary to document the expected behavior of the public interface with tests.
-</details>
-
-<details>
-  <summary>Vuex actions</summary>
-  Any Vuex action needs to work in a consistent way independent of the component it is triggered from.
-</details>
-
-<details>
-  <summary>Vuex mutations</summary>
-  For complex Vuex mutations it helps to identify the source of a problem by separating the tests from other parts of the Vuex store.
-</details>
-
-#### When *not* to use unit tests
-
-<details>
-  <summary>non-exported functions or classes</summary>
-  Anything that is not exported from a module can be considered private or an implementation detail and doesn't need to be tested.
-</details>
-
-<details>
-  <summary>constants</summary>
-  Testing the value of a constant would mean to copy it.
-  This results in extra effort without additional confidence that the value is correct.
-</details>
-
-<details>
-  <summary>Vue components</summary>
-  Computed properties, methods, and lifecycle hooks can be considered an implementation detail of components and don't need to be tested.
-  They are implicitly covered by component tests.
-  The <a href="https://vue-test-utils.vuejs.org/guides/#getting-started">official Vue guidelines</a> suggest the same.
-</details>
-
-#### What to mock in unit tests
-
-<details>
-  <summary>state of the class under test</summary>
-  Modifying the state of the class under test directly rather than using methods of the class avoids side-effects in test setup.
-</details>
-
-<details>
-  <summary>other exported classes</summary>
-  Every class needs to be tested in isolation to prevent test scenarios from growing exponentially.
-</details>
-
-<details>
-  <summary>single DOM elements if passed as parameters</summary>
-  For tests that only operate on single DOM elements rather than a whole page, creating these elements is cheaper than loading a whole HTML fixture.
-</details>
-
-<details>
-  <summary>all server requests</summary>
-  When running frontend unit tests, the backend may not be reachable.
-  Therefore all outgoing requests need to be mocked.
-</details>
-
-<details>
-  <summary>asynchronous background operations</summary>
-  Background operations cannot be stopped or waited on, so they will continue running in the following tests and cause side effects.
-</details>
-
-#### What *not* to mock in unit tests
-
-<details>
-  <summary>non-exported functions or classes</summary>
-  Everything that is not exported can be considered private to the module and will be implicitly tested via the exported classes / functions.
-</details>
-
-<details>
-  <summary>methods of the class under test</summary>
-  By mocking methods of the class under test, the mocks will be tested and not the real methods.
-</details>
-
-<details>
-  <summary>utility functions (pure functions, or those that only modify parameters)</summary>
-  If a function has no side effects because it has no state, it is safe to not mock it in tests.
-</details>
-
-<details>
-  <summary>full HTML pages</summary>
-  Loading the HTML of a full page slows down tests, so it should be avoided in unit tests.
-</details>
-
-### Frontend component tests
-
-Component tests cover the state of a single component that is perceivable by a user depending on external signals such as user input, events fired from other components, or application state.
-
-```mermaid
-graph RL
-    plain[Plain JavaScript];
-    Vue[Vue Components];
-    feature-flags[Feature Flags];
-    license-checks[License Checks];
-
-    plain---Vuex;
-    plain---GraphQL;
-    Vue---plain;
-    Vue---Vuex;
-    Vue---GraphQL;
-    browser---plain;
-    browser---Vue;
-    plain---backend;
-    Vuex---backend;
-    GraphQL---backend;
-    Vue---backend;
-    backend---database;
-    backend---feature-flags;
-    backend---license-checks;
-
-    class Vue tested;
-
-    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
-    classDef label stroke-width:0;
-    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
-
-    subgraph " "
-    tested;
-    mocked;
-    class tested tested;
-    end
-```
-
-#### When to use component tests
-
-- Vue components
-
-#### When *not* to use component tests
-
-<details>
-  <summary>Vue applications</summary>
-  Vue applications may contain many components.
-  Testing them on a component level requires too much effort.
-  Therefore they are tested on frontend integration level.
-</details>
-
-<details>
-  <summary>HAML templates</summary>
-  HAML templates contain only Markup and no frontend-side logic.
-  Therefore they are not complete components.
-</details>
-
-#### What to mock in component tests
-
-<details>
-  <summary>DOM</summary>
-  Operating on the real DOM is significantly slower than on the virtual DOM.
-</details>
-
-<details>
-  <summary>properties and state of the component under test</summary>
-  Similarly to testing classes, modifying the properties directly (rather than relying on methods of the component) avoids side-effects.
-</details>
-
-<details>
-  <summary>Vuex store</summary>
-  To avoid side effects and keep component tests simple, Vuex stores are replaced with mocks.
-</details>
-
-<details>
-  <summary>all server requests</summary>
-  Similar to unit tests, when running component tests, the backend may not be reachable.
-  Therefore all outgoing requests need to be mocked.
-</details>
-
-<details>
-  <summary>asynchronous background operations</summary>
-  Similar to unit tests, background operations cannot be stopped or waited on, so they will continue running in the following tests and cause side effects.
-</details>
-
-<details>
-  <summary>child components</summary>
-  Every component is tested individually, so child components are mocked.
-  See also <a href="https://vue-test-utils.vuejs.org/api/#shallowmount">shallowMount()</a>
-</details>
-
-#### What *not* to mock in component tests
-
-<details>
-  <summary>methods or computed properties of the component under test</summary>
-  By mocking part of the component under test, the mocks will be tested and not the real component.
-</details>
-
-<details>
-  <summary>functions and classes independent from Vue</summary>
-  All plain JavaScript code is already covered by unit tests and needs not to be mocked in component tests.
-</details>
-
-### Frontend integration tests
-
-Integration tests cover the interaction between all components on a single page.
-Their abstraction level is comparable to how a user would interact with the UI.
-
-```mermaid
-graph RL
-    plain[Plain JavaScript];
-    Vue[Vue Components];
-    feature-flags[Feature Flags];
-    license-checks[License Checks];
-
-    plain---Vuex;
-    plain---GraphQL;
-    Vue---plain;
-    Vue---Vuex;
-    Vue---GraphQL;
-    browser---plain;
-    browser---Vue;
-    plain---backend;
-    Vuex---backend;
-    GraphQL---backend;
-    Vue---backend;
-    backend---database;
-    backend---feature-flags;
-    backend---license-checks;
-
-    class plain tested;
-    class Vue tested;
-    class Vuex tested;
-    class GraphQL tested;
-    class browser tested;
-    linkStyle 0,1,2,3,4,5,6 stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
-
-    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
-    classDef label stroke-width:0;
-    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
-
-    subgraph " "
-    tested;
-    mocked;
-    class tested tested;
-    end
-```
-
-#### When to use integration tests
-
-<details>
-  <summary>page bundles (<code>index.js</code> files in <code>app/assets/javascripts/pages/</code>)</summary>
-  Testing the page bundles ensures the corresponding frontend components integrate well.
-</details>
-
-<details>
-  <summary>Vue applications outside of page bundles</summary>
-  Testing Vue applications as a whole ensures the corresponding frontend components integrate well.
-</details>
-
-#### What to mock in integration tests
-
-<details>
-  <summary>HAML views (use fixtures instead)</summary>
-  Rendering HAML views requires a Rails environment including a running database which we cannot rely on in frontend tests.
-</details>
-
-<details>
-  <summary>all server requests</summary>
-  Similar to unit and component tests, when running component tests, the backend may not be reachable.
-  Therefore all outgoing requests need to be mocked.
-</details>
-
-<details>
-  <summary>asynchronous background operations that are not perceivable on the page</summary>
-  Background operations that affect the page need to be tested on this level.
-  All other background operations cannot be stopped or waited on, so they will continue running in the following tests and cause side effects.
-</details>
-
-#### What *not* to mock in integration tests
-
-<details>
-  <summary>DOM</summary>
-  Testing on the real DOM ensures our components work in the environment they are meant for.
-  Part of this will be delegated to <a href="https://gitlab.com/gitlab-org/quality/team-tasks/issues/45">cross-browser testing</a>.
-</details>
-
-<details>
-  <summary>properties or state of components</summary>
-  On this level, all tests can only perform actions a user would do.
-  For example to change the state of a component, a click event would be fired.
-</details>
-
-<details>
-  <summary>Vuex stores</summary>
-  When testing the frontend code of a page as a whole, the interaction between Vue components and Vuex stores is covered as well.
-</details>
-
-### Feature tests
-
-In contrast to [frontend integration tests](#frontend-integration-tests), feature tests make requests against the real backend instead of using fixtures.
-This also implies that database queries are executed which makes this category significantly slower.
-
-See also
-
-- The [RSpec testing guidelines](../testing_guide/best_practices.md#rspec).
-- System / Feature tests in the [Testing Best Practices](best_practices.md#system--feature-tests).
-- [Issue #26159](https://gitlab.com/gitlab-org/gitlab/issues/26159) which aims at combine those guidelines with this page.
-
-```mermaid
-graph RL
-    plain[Plain JavaScript];
-    Vue[Vue Components];
-    feature-flags[Feature Flags];
-    license-checks[License Checks];
-
-    plain---Vuex;
-    plain---GraphQL;
-    Vue---plain;
-    Vue---Vuex;
-    Vue---GraphQL;
-    browser---plain;
-    browser---Vue;
-    plain---backend;
-    Vuex---backend;
-    GraphQL---backend;
-    Vue---backend;
-    backend---database;
-    backend---feature-flags;
-    backend---license-checks;
-
-    class backend tested;
-    class plain tested;
-    class Vue tested;
-    class Vuex tested;
-    class GraphQL tested;
-    class browser tested;
-    linkStyle 0,1,2,3,4,5,6,7,8,9,10 stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
-
-    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
-    classDef label stroke-width:0;
-    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
-
-    subgraph " "
-    tested;
-    mocked;
-    class tested tested;
-    end
-```
-
-#### When to use feature tests
-
-- Use cases that require a backend and cannot be tested using fixtures.
-- Behavior that is not part of a page bundle but defined globally.
-
-#### Relevant notes
-
-A `:js` flag is added to the test to make sure the full environment is loaded.
-
-```ruby
-scenario 'successfully', :js do
-  sign_in(create(:admin))
-end
-```
-
-The steps of each test are written using capybara methods ([documentation](https://www.rubydoc.info/gems/capybara)).
-
-Bear in mind <abbr title="XMLHttpRequest">XHR</abbr> calls might require you to use `wait_for_requests` in between steps, like so:
-
-```ruby
-find('.form-control').native.send_keys(:enter)
-
-wait_for_requests
-
-expect(page).not_to have_selector('.card')
-```
-
 ## Test helpers
 
 ### Vuex Helper: `testAction`
diff --git a/doc/development/testing_guide/testing_levels.md b/doc/development/testing_guide/testing_levels.md
index 13659d66180..f7dec82724f 100644
--- a/doc/development/testing_guide/testing_levels.md
+++ b/doc/development/testing_guide/testing_levels.md
@@ -51,6 +51,167 @@ records should use stubs/doubles as much as possible.
 | `rubocop/` | `spec/rubocop/` | RSpec | |
 | `spec/factories` | `spec/factories_spec.rb` | RSpec | |
 
+### Frontend unit tests
+
+Unit tests are on the lowest abstraction level and typically test functionality
+that is not directly perceivable by a user.
+
+```mermaid
+graph RL
+    plain[Plain JavaScript];
+    Vue[Vue Components];
+    feature-flags[Feature Flags];
+    license-checks[License Checks];
+
+    plain---Vuex;
+    plain---GraphQL;
+    Vue---plain;
+    Vue---Vuex;
+    Vue---GraphQL;
+    browser---plain;
+    browser---Vue;
+    plain---backend;
+    Vuex---backend;
+    GraphQL---backend;
+    Vue---backend;
+    backend---database;
+    backend---feature-flags;
+    backend---license-checks;
+
+    class plain tested;
+    class Vuex tested;
+
+    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
+    classDef label stroke-width:0;
+    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
+
+    subgraph " "
+    tested;
+    mocked;
+    class tested tested;
+    end
+```
+
+#### When to use unit tests
+
+- **Exported functions and classes**:
+  Anything exported can be reused at various places in ways you have no control over.
+  You should document the expected behavior of the public interface with tests.
+- **Vuex actions**:
+  Any Vuex action must work in a consistent way, independent of the component it is triggered from.
+- **Vuex mutations**:
+  For complex Vuex mutations, you should separate the tests from other parts of the Vuex store to simplify problem-solving.
+
+#### When *not* to use unit tests
+  
+- **Non-exported functions or classes**:
+  Anything not exported from a module can be considered private or an implementation detail, and doesn't need to be tested.
+- **Constants**:
+  Testing the value of a constant means copying it, resulting in extra effort without additional confidence that the value is correct.
+- **Vue components**:
+  Computed properties, methods, and lifecycle hooks can be considered an implementation detail of components, are implicitly covered by component tests, and don't need to be tested.
+  For more information, see the [official Vue guidelines](https://vue-test-utils.vuejs.org/guides/#getting-started).
+
+#### What to mock in unit tests
+
+- **State of the class under test**:
+  Modifying the state of the class under test directly rather than using methods of the class avoids side effects in test setup.
+- **Other exported classes**:
+  Every class must be tested in isolation to prevent test scenarios from growing exponentially.
+- **Single DOM elements if passed as parameters**:
+  For tests only operating on single DOM elements, rather than a whole page, creating these elements is cheaper than loading an entire HTML fixture.
+- **All server requests**:
+  When running frontend unit tests, the backend may not be reachable, so all outgoing requests need to be mocked.
+- **Asynchronous background operations**:
+  Background operations cannot be stopped or waited on, so they will continue running in the following tests and cause side effects.
+
+#### What *not* to mock in unit tests
+
+- **Non-exported functions or classes**:
+  Everything that is not exported can be considered private to the module, and will be implicitly tested through the exported classes and functions.
+- **Methods of the class under test**:
+  By mocking methods of the class under test, the mocks will be tested and not the real methods.
+- **Utility functions (pure functions, or those that only modify parameters)**:
+ If a function has no side effects because it has no state, it is safe to not mock it in tests.
+- **Full HTML pages**:
+  Avoid loading the HTML of a full page in unit tests, as it slows down tests.
+
+### Frontend component tests
+
+Component tests cover the state of a single component that is perceivable by a user depending on external signals such as user input, events fired from other components, or application state.
+
+```mermaid
+graph RL
+    plain[Plain JavaScript];
+    Vue[Vue Components];
+    feature-flags[Feature Flags];
+    license-checks[License Checks];
+
+    plain---Vuex;
+    plain---GraphQL;
+    Vue---plain;
+    Vue---Vuex;
+    Vue---GraphQL;
+    browser---plain;
+    browser---Vue;
+    plain---backend;
+    Vuex---backend;
+    GraphQL---backend;
+    Vue---backend;
+    backend---database;
+    backend---feature-flags;
+    backend---license-checks;
+
+    class Vue tested;
+
+    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
+    classDef label stroke-width:0;
+    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
+
+    subgraph " "
+    tested;
+    mocked;
+    class tested tested;
+    end
+```
+
+#### When to use component tests
+
+- **Vue components**
+
+#### When *not* to use component tests
+
+- **Vue applications**:
+  Vue applications may contain many components.
+  Testing them on a component level requires too much effort.
+  Therefore they are tested on frontend integration level.
+- **HAML templates**:
+  HAML templates contain only Markup and no frontend-side logic.
+  Therefore they are not complete components.
+
+#### What to mock in component tests
+
+- **DOM**:
+  Operating on the real DOM is significantly slower than on the virtual DOM.
+- **Properties and state of the component under test**:
+  Similar to testing classes, modifying the properties directly (rather than relying on methods of the component) avoids side effects.
+- **Vuex store**:
+  To avoid side effects and keep component tests simple, Vuex stores are replaced with mocks.
+- **All server requests**:
+  Similar to unit tests, when running component tests, the backend may not be reachable, so all outgoing requests need to be mocked.
+- **Asynchronous background operations**:
+  Similar to unit tests, background operations cannot be stopped or waited on, so they will continue running in the following tests and cause side effects.
+- **Child components**:
+  Every component is tested individually, so child components are mocked.
+  See also [`shallowMount()`](https://vue-test-utils.vuejs.org/api/#shallowmount)
+
+#### What *not* to mock in component tests
+
+- **Methods or computed properties of the component under test**:
+  By mocking part of the component under test, the mocks will be tested and not the real component.
+- **Functions and classes independent from Vue**:
+  All plain JavaScript code is already covered by unit tests and needs not to be mocked in component tests.
+
 ## Integration tests
 
 Formal definition: <https://en.wikipedia.org/wiki/Integration_testing>
@@ -66,14 +227,86 @@ They're useful to test permissions, redirections, what view is rendered etc.
 | `app/controllers/` | `spec/controllers/` | RSpec | For N+1 tests, use [request specs](../query_recorder.md#use-request-specs-instead-of-controller-specs) |
 | `app/mailers/` | `spec/mailers/` | RSpec | |
 | `lib/api/` | `spec/requests/api/` | RSpec | |
-| `app/assets/javascripts/` | `spec/javascripts/`, `spec/frontend/` | Karma & Jest | More details in the [Frontend Testing guide](frontend_testing.md) section. |
+| `app/assets/javascripts/` | `spec/javascripts/`, `spec/frontend/` | Karma & Jest | [More details below](#frontend-integration-tests) |
+
+### Frontend integration tests
+
+Integration tests cover the interaction between all components on a single page.
+Their abstraction level is comparable to how a user would interact with the UI.
+
+```mermaid
+graph RL
+    plain[Plain JavaScript];
+    Vue[Vue Components];
+    feature-flags[Feature Flags];
+    license-checks[License Checks];
+
+    plain---Vuex;
+    plain---GraphQL;
+    Vue---plain;
+    Vue---Vuex;
+    Vue---GraphQL;
+    browser---plain;
+    browser---Vue;
+    plain---backend;
+    Vuex---backend;
+    GraphQL---backend;
+    Vue---backend;
+    backend---database;
+    backend---feature-flags;
+    backend---license-checks;
+
+    class plain tested;
+    class Vue tested;
+    class Vuex tested;
+    class GraphQL tested;
+    class browser tested;
+    linkStyle 0,1,2,3,4,5,6 stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
+
+    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
+    classDef label stroke-width:0;
+    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
+
+    subgraph " "
+    tested;
+    mocked;
+    class tested tested;
+    end
+```
+
+#### When to use integration tests
+
+- **Page bundles (`index.js` files in `app/assets/javascripts/pages/`)**:
+  Testing the page bundles ensures the corresponding frontend components integrate well.
+- **Vue applications outside of page bundles**:
+  Testing Vue applications as a whole ensures the corresponding frontend components integrate well.
+
+#### What to mock in integration tests
+
+- **HAML views (use fixtures instead)**:
+  Rendering HAML views requires a Rails environment including a running database, which you cannot rely on in frontend tests.
+- **All server requests**:
+  Similar to unit and component tests, when running component tests, the backend may not be reachable, so all outgoing requests must be mocked.
+- **Asynchronous background operations that are not perceivable on the page**:
+  Background operations that affect the page must be tested on this level.
+  All other background operations cannot be stopped or waited on, so they will continue running in the following tests and cause side effects.
+
+#### What *not* to mock in integration tests
+
+- **DOM**:
+  Testing on the real DOM ensures your components work in the intended environment.
+  Part of DOM testing is delegated to [cross-browser testing](https://gitlab.com/gitlab-org/quality/team-tasks/issues/45).
+- **Properties or state of components**:
+  On this level, all tests can only perform actions a user would do.
+  For example: to change the state of a component, a click event would be fired.
+- **Vuex stores**:
+  When testing the frontend code of a page as a whole, the interaction between Vue components and Vuex stores is covered as well.
 
 ### About controller tests
 
 In an ideal world, controllers should be thin. However, when this is not the
-case, it's acceptable to write a system/feature test without JavaScript instead
-of a controller test. The reason is that testing a fat controller usually
-involves a lot of stubbing, things like:
+case, it's acceptable to write a system or feature test without JavaScript instead
+of a controller test. Testing a fat controller usually involves a lot of stubbing, such as:
 
 ```ruby
 controller.instance_variable_set(:@user, user)
@@ -85,8 +318,7 @@ and use methods which are deprecated in Rails 5 ([#23768]).
 
 ### About Karma
 
-As you may have noticed, Karma is both in the Unit tests and the Integration
-tests category. That's because Karma is a tool that provides an environment to
+Karma is both in the Unit tests and the Integration tests category. Karma provides an environment to
 run JavaScript tests, so you can either run unit tests (e.g. test a single
 JavaScript method), or integration tests (e.g. test a component that is composed
 of multiple components).
@@ -98,7 +330,7 @@ Formal definitions:
 - <https://en.wikipedia.org/wiki/System_testing>
 - <https://en.wikipedia.org/wiki/White-box_testing>
 
-These kind of tests ensure the GitLab *Rails* application (i.e.
+These kind of tests ensure the GitLab *Rails* application (for example,
 `gitlab-foss`/`gitlab`) works as expected from a *browser* point of view.
 
 Note that:
@@ -118,14 +350,94 @@ makes sense since it's a small component, which cannot be tested at the unit or
 controller level.
 
 Only test the happy path, but make sure to add a test case for any regression
-that couldn't have been caught at lower levels with better tests (i.e. if a
-regression is found, regression tests should be added at the lowest-level
+that couldn't have been caught at lower levels with better tests (for example, if a
+regression is found, regression tests should be added at the lowest level
 possible).
 
 | Tests path | Testing engine | Notes |
 | ---------- | -------------- | ----- |
 | `spec/features/` | [Capybara] + [RSpec] | If your test has the `:js` metadata, the browser driver will be [Poltergeist], otherwise it's using [RackTest]. |
 
+### Frontend feature tests
+
+In contrast to [frontend integration tests](#frontend-integration-tests), feature
+tests make requests against the real backend instead of using fixtures.
+This also implies that database queries are executed which makes this category significantly slower.
+
+See also:
+
+- The [RSpec testing guidelines](../testing_guide/best_practices.md#rspec).
+- System / Feature tests in the [Testing Best Practices](best_practices.md#system--feature-tests).
+- [Issue #26159](https://gitlab.com/gitlab-org/gitlab/issues/26159) which aims at combining those guidelines with this page.
+
+```mermaid
+graph RL
+    plain[Plain JavaScript];
+    Vue[Vue Components];
+    feature-flags[Feature Flags];
+    license-checks[License Checks];
+
+    plain---Vuex;
+    plain---GraphQL;
+    Vue---plain;
+    Vue---Vuex;
+    Vue---GraphQL;
+    browser---plain;
+    browser---Vue;
+    plain---backend;
+    Vuex---backend;
+    GraphQL---backend;
+    Vue---backend;
+    backend---database;
+    backend---feature-flags;
+    backend---license-checks;
+
+    class backend tested;
+    class plain tested;
+    class Vue tested;
+    class Vuex tested;
+    class GraphQL tested;
+    class browser tested;
+    linkStyle 0,1,2,3,4,5,6,7,8,9,10 stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
+
+    classDef node color:#909090,fill:#f0f0f0,stroke-width:2px,stroke:#909090
+    classDef label stroke-width:0;
+    classDef tested color:#000000,fill:#a0c0ff,stroke:#6666ff,stroke-width:2px,stroke-dasharray: 5, 5;
+
+    subgraph " "
+    tested;
+    mocked;
+    class tested tested;
+    end
+```
+
+#### When to use feature tests
+
+- Use cases that require a backend, and cannot be tested using fixtures.
+- Behavior that is not part of a page bundle, but defined globally.
+
+#### Relevant notes
+
+A `:js` flag is added to the test to make sure the full environment is loaded:
+
+```ruby
+scenario 'successfully', :js do
+  sign_in(create(:admin))
+end
+```
+
+The steps of each test are written using ([capybara methods](https://www.rubydoc.info/gems/capybara)).
+
+XHR (XMLHttpRequest) calls might require you to use `wait_for_requests` in between steps, such as:
+
+```ruby
+find('.form-control').native.send_keys(:enter)
+
+wait_for_requests
+
+expect(page).not_to have_selector('.card')
+```
+
 ### Consider **not** writing a system test
 
 If we're confident that the low-level components work well (and we should be if