improve signing form header

app/javascript/elements/scroll_buttons.js

@@ -0,0 +1,56 @@
+export default class extends HTMLElement {
+  connectedCallback () {
+    this.header = document.querySelector('#signing_form_header')
+
+    window.addEventListener('scroll', this.onScroll.bind(this))
+    window.addEventListener('resize', this.onResize.bind(this))
+
+    this.onResize()
+  }
+
+  disconnectedCallback () {
+    window.removeEventListener('scroll', this.onScroll.bind(this))
+    window.removeEventListener('resize', this.onResize.bind(this))
+  }
+
+  onResize () {
+    if (this.isNarrow()) {
+      this.hideButtons(true)
+    } else if (this.isHeaderNotVisible()) {
+      this.showButtons()
+    }
+  }
+
+  isNarrow () {
+    return window.innerWidth < 1366
+  }
+
+  onScroll () {
+    if (this.isHeaderNotVisible() && !this.isNarrow()) {
+      this.showButtons()
+    } else {
+      this.hideButtons()
+    }
+  }
+
+  isHeaderNotVisible () {
+    const rect = this.header.getBoundingClientRect()
+    return rect.bottom <= 0 || rect.top >= window.innerHeight
+  }
+
+  showButtons () {
+    this.classList.remove('hidden', '-translate-y-10', 'opacity-0')
+    this.classList.add('translate-y-0', 'opacity-100')
+  }
+
+  hideButtons () {
+    this.classList.remove('translate-y-0', 'opacity-100')
+    this.classList.add('-translate-y-10', 'opacity-0')
+
+    setTimeout(() => {
+      if (this.classList.contains('-translate-y-10')) {
+        this.classList.add('hidden')
+      }
+    }, 300)
+  }
+}