Adds CSS-based skeleton loader

What does this MR do?

Adds a gl-animation-skeleton-loader utility class to create skeleton animations.

See Draft: GitLab UI integration branch for kpalchy... (gitlab!129285 - closed) for integration test and a example / comparison with previous <gl-skeleton-loader> solution.

Currently common utility to use for "skeleton loading" is a <gl-skeleton-loader> component, that works with SVG:

<gl-skeleton-loader :width="340" :height="100">
  <rect width="340" height="16" rx="4" />
  <rect y="30" width="118" height="16" rx="8" />
  <rect x="122" y="30" width="130" height="16" rx="8" />
  <rect y="62" width="38" height="16" rx="4" />
  <circle cx="320" cy="68" r="16" />
</gl-skeleton-loader>

But there are a few problems with it:

• we can't use familiar css utils to represent HTML we're trying to mock, e.g. for gl-rounded-full we need a <circle />
• because SVG it's not as responsive as HTML/CSS is (it scales instead of adapting, + there's no gl-lg-display-none utils)
• it's hard to read: all the x/y coordinates need to be calculated

With css-based animation we won't need a dedicated component, we'll be able to use familiar css utils, and it will be responsive. See screenshots section for more.

Screenshots or screen recordings

Simple example:

<div class="gl-display-flex gl-flex-direction-column gl-gap-2">
  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2"></div>
  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2"></div>
  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2"></div>
</div>

A more complex example:

<div class="gl-display-flex gl-flex-direction-column gl-gap-2 gl-w-30">
  <div class="gl-animate-skeleton-loader gl-h-8 gl-rounded-base gl-mb-4"></div>
  <div class="gl-display-flex gl-flex-direction-row gl-gap-2">
    <div class="gl-animate-skeleton-loader gl-h-8 gl-w-8 gl-rounded-full"></div>

    <div class="gl-flex-grow-1" style="animation-delay: 100ms">
      <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2"></div>
      <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2"></div>
      <div class="gl-animate-skeleton-loader gl-display-inline-block gl-h-4 gl-w-10 gl-rounded-base gl-my-2"></div>
      <div class="gl-animate-skeleton-loader gl-display-inline-block gl-h-4 gl-w-10 gl-rounded-base gl-my-2" style="animation-delay: 250ms"></div>
    </div>
  </div>
</div>

Testing the animation

Apply this patch

patch

diff --git a/src/components/base/skeleton_loader/skeleton_loader.scss b/src/components/base/skeleton_loader/skeleton_loader.scss
index 6850a395..7cb2ce74 100644
--- a/src/components/base/skeleton_loader/skeleton_loader.scss
+++ b/src/components/base/skeleton_loader/skeleton_loader.scss
@@ -15,3 +15,8 @@
     @include gl-stop-color-gray-50;
   }
 }
+
+.gl-animate-skeleton-loader {
+  background-color: red;
+  background-image: linear-gradient(to right, red 0, blue 23%, blue 27%, red 50%);
+}
\ No newline at end of file
diff --git a/src/components/base/skeleton_loader/skeleton_loader.stories.js b/src/components/base/skeleton_loader/skeleton_loader.stories.js
index a219088e..7182a8f0 100644
--- a/src/components/base/skeleton_loader/skeleton_loader.stories.js
+++ b/src/components/base/skeleton_loader/skeleton_loader.stories.js
@@ -2,6 +2,41 @@ import { makeContainer } from '../../../utils/story_decorators/container';
 import GlSkeletonLoader from './skeleton_loader.vue';
 import readme from './skeleton_loader.md';
 
+export const CssTest1 = () => ({
+template: `
+<div class="gl-display-flex gl-flex-direction-column gl-gap-2">
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 25px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 50px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 100px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 150px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 200px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 250px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 300px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 350px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 400px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 450px;"></div>
+  <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2" style="width: 500px;"></div>
+</div>
+`
+})
+
+export const CssTest2 = () => ({
+template: `
+<div class="gl-display-flex gl-flex-direction-column gl-gap-2 gl-w-30">
+  <div class="gl-animate-skeleton-loader gl-h-8 gl-rounded-base gl-mb-4"></div>
+  <div class="gl-display-flex gl-flex-direction-row gl-gap-2">
+    <div class="gl-animate-skeleton-loader gl-h-8 gl-w-8 gl-rounded-full"></div>
+    <div class="gl-flex-grow-1" style="animation-delay: 100ms">
+      <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2"></div>
+      <div class="gl-animate-skeleton-loader gl-h-4 gl-rounded-base gl-my-2"></div>
+      <div class="gl-animate-skeleton-loader gl-display-inline-block gl-h-4 gl-w-10 gl-rounded-base gl-my-2"></div>
+      <div class="gl-animate-skeleton-loader gl-display-inline-block gl-h-4 gl-w-10 gl-rounded-base gl-my-2" style="animation-delay: 250ms"></div>
+    </div>
+  </div>
+</div>
+`
+})
+
 const defaultValue = (prop) => GlSkeletonLoader.props[prop].default;
 
 const generateProps = ({

Run npm run storybook, open http://localhost:9001/?path=/docs/base-skeleton-loader--docs

Integration merge requests

• GitLab: gitlab!129285 (closed) (for test only)
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

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Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

This is kinda like https://xkcd.com/927/ ...