add mobile filters

app/javascript/application.js

@@ -56,6 +56,7 @@ import OpenModal from './elements/open_modal'
 import BarChart from './elements/bar_chart'
 import FieldCondition from './elements/field_condition'
 import ConfirmUpload from './elements/confirm_upload'
+import ScrollFade from './elements/scroll_fade'
 
 import * as TurboInstantClick from './lib/turbo_instant_click'
 
@@ -150,6 +151,7 @@ safeRegisterElement('open-modal', OpenModal)
 safeRegisterElement('bar-chart', BarChart)
 safeRegisterElement('field-condition', FieldCondition)
 safeRegisterElement('confirm-upload', ConfirmUpload)
+safeRegisterElement('scroll-fade', ScrollFade)
 
 safeRegisterElement('template-builder', class extends HTMLElement {
   connectedCallback () {

app/javascript/elements/scroll_fade.js

@@ -0,0 +1,40 @@
+export default class extends HTMLElement {
+  connectedCallback () {
+    this.addEventListener('scroll', this.updateFade, { passive: true })
+    window.addEventListener('resize', this.updateFade)
+
+    this.updateFade()
+  }
+
+  disconnectedCallback () {
+    window.removeEventListener('resize', this.updateFade)
+  }
+
+  updateFade = () => {
+    const maxScroll = this.scrollWidth - this.clientWidth
+    const fadeGradient = {
+      right: 'linear-gradient(to right, black calc(100% - 24px), transparent)',
+      left: 'linear-gradient(to right, transparent, black 24px)',
+      both: 'linear-gradient(to right, transparent, black 24px, black calc(100% - 24px), transparent)'
+    }
+
+    let state = 'none'
+
+    if (maxScroll > 4) {
+      if (this.scrollLeft <= 4) {
+        state = 'right'
+      } else if (this.scrollLeft >= maxScroll - 4) {
+        state = 'left'
+      } else {
+        state = 'both'
+      }
+    }
+
+    this.applyFade(fadeGradient[state] || '')
+  }
+
+  applyFade (gradient) {
+    this.style.maskImage = gradient
+    this.style.webkitMaskImage = gradient
+  }
+}