feat(www): add 'lead' animation to the Ecosystem horizontal scrollers

www/src/components/ecosystem/ecosystem-board.js

@@ -1,4 +1,4 @@
-import React from "react"
+import React, { Component } from "react"
 import PropTypes from "prop-types"
 import styled from "react-emotion"
 
@@ -20,46 +20,116 @@ const EcosystemBoardRoot = styled(`div`)`
   }
 `
 
-const EcosystemBoard = ({
-  icons: { plugins: PluginsIcon, starters: StartersIcon },
-  starters,
-  plugins,
-}) => (
-  <EcosystemBoardRoot>
-    <EcosystemSection
-      title="Plugins"
-      description="Plugins are packages that extend Gatsby sites. They can source content, transform data, and more!"
-      subTitle="Featured Plugins"
-      icon={PluginsIcon}
-      links={[
-        { label: `Browse Plugins`, to: `/plugins/` },
-        {
-          label: `Plugin Authoring`,
-          to: `/docs/plugin-authoring/`,
-          secondary: true,
-        },
-        { label: `Plugin Docs`, to: `/docs/plugins/`, secondary: true },
-      ]}
-      featuredItems={plugins}
-    />
-    <EcosystemSection
-      title="Starters"
-      description="Starters are Gatsby sites that are preconfigured for different use cases to give you a head start for your project."
-      subTitle="Featured Starters"
-      icon={StartersIcon}
-      links={[
-        { label: `Browse Starters`, to: `/starters/` },
-        { label: `Starter Docs`, to: `/docs/starters/`, secondary: true },
-      ]}
-      featuredItems={starters}
-    />
-    <EcosystemSection
-      title="External Resources"
-      description="A curated list of interesting Gatsby community projects and learning resources like podcasts and tutorials."
-      links={[{ label: `Browse Resources`, to: `/docs/awesome-gatsby/` }]}
-    />
-  </EcosystemBoardRoot>
-)
+class EcosystemBoard extends Component {
+  observer
+  observerTargets = []
+
+  componentDidMount() {
+    if (typeof window.IntersectionObserver !== `undefined`) {
+      this.setupObserver()
+    }
+  }
+
+  componentWillUnmount() {
+    if (typeof window.IntersectionObserver !== `undefined`) {
+      this.observerTargets.forEach(target => this.observer.unobserve(target))
+    }
+  }
+
+  setupObserver = () => {
+    const options = { rootMargin: `0px`, threshold: [1] }
+    this.observer = new IntersectionObserver(this.handleIntersect, options)
+    this.observerTargets = Array.from(
+      document.querySelectorAll(`.featuredItems`)
+    )
+
+    this.observerTargets.forEach(target => this.observer.observe(target))
+  }
+
+  handleIntersect = (entries, observer) => {
+    entries.forEach(entry => {
+      const target = entry.target
+
+      if (entry.intersectionRatio > 0) {
+        setTimeout(
+          () => this.turnOnLeadScroll({ target, duration: 1000, distance: 20 }),
+          250
+        )
+        this.observer.unobserve(target)
+      }
+    })
+  }
+
+  turnOnLeadScroll = ({ target, duration, distance }) => {
+    let startTime = null
+
+    function animation(currentTime) {
+      if (startTime === null) {
+        startTime = currentTime
+      }
+
+      const timeElapsed = currentTime - startTime
+      const getDistanceToScroll = ease(timeElapsed, 0, distance, duration)
+
+      target.scroll({ top: 0, left: getDistanceToScroll })
+
+      if (timeElapsed < duration) {
+        requestAnimationFrame(animation)
+      }
+    }
+
+    function ease(t, b, c, d) {
+      return -c * (t /= d) * (t - 2) + b
+    }
+
+    requestAnimationFrame(animation)
+  }
+
+  render() {
+    const {
+      icons: { plugins: PluginsIcon, starters: StartersIcon },
+      starters,
+      plugins,
+    } = this.props
+
+    return (
+      <EcosystemBoardRoot>
+        <EcosystemSection
+          title="Plugins"
+          description="Plugins are packages that extend Gatsby sites. They can source content, transform data, and more!"
+          subTitle="Featured Plugins"
+          icon={PluginsIcon}
+          links={[
+            { label: `Browse Plugins`, to: `/plugins/` },
+            {
+              label: `Plugin Authoring`,
+              to: `/docs/plugin-authoring/`,
+              secondary: true,
+            },
+            { label: `Plugin Docs`, to: `/docs/plugins/`, secondary: true },
+          ]}
+          featuredItems={plugins}
+        />
+        <EcosystemSection
+          title="Starters"
+          description="Starters are Gatsby sites that are preconfigured for different use cases to give you a head start for your project."
+          subTitle="Featured Starters"
+          icon={StartersIcon}
+          links={[
+            { label: `Browse Starters`, to: `/starters/` },
+            { label: `Starter Docs`, to: `/docs/starters/`, secondary: true },
+          ]}
+          featuredItems={starters}
+        />
+        <EcosystemSection
+          title="External Resources"
+          description="A curated list of interesting Gatsby community projects and learning resources like podcasts and tutorials."
+          links={[{ label: `Browse Resources`, to: `/docs/awesome-gatsby/` }]}
+        />
+      </EcosystemBoardRoot>
+    )
+  }
+}
 
 EcosystemBoard.propTypes = {
   icons: PropTypes.object,

www/src/components/ecosystem/ecosystem-featured-items.js

@@ -11,6 +11,7 @@ import { scrollbarStyles } from "../../utils/styles"
 const EcosystemFeaturedItemsRoot = styled(`div`)`
   overflow-x: scroll;
   margin: ${rhythm(0.1)} -${rhythm(options.blockMarginBottom)};
+  -webkit-overflow-scrolling: touch;
 
   ${presets.Tablet} {
     border-top: 1px solid ${colors.gray.superLight};
@@ -37,7 +38,7 @@ const List = styled(`ul`)`
 `
 
 const EcosystemFeaturedItems = ({ items }) => (
-  <EcosystemFeaturedItemsRoot>
+  <EcosystemFeaturedItemsRoot className="featuredItems">
     <List numberOfItems={items.length}>
       {items.map(item => {
         const { slug } = item