add default user signatures

app/javascript/elements/signature_form.js

@@ -0,0 +1,46 @@
+import { target, targetable } from '@github/catalyst/lib/targetable'
+import { cropCanvasAndExportToPNG } from '../submission_form/crop_canvas'
+
+export default targetable(class extends HTMLElement {
+  static [target.static] = ['canvas', 'input', 'clear', 'button']
+
+  async connectedCallback () {
+    this.canvas.width = this.canvas.parentNode.parentNode.clientWidth
+    this.canvas.height = this.canvas.parentNode.parentNode.clientWidth / 3
+
+    const { default: SignaturePad } = await import('signature_pad')
+
+    this.pad = new SignaturePad(this.canvas)
+
+    this.clear.addEventListener('click', (e) => {
+      e.preventDefault()
+
+      this.pad.clear()
+    })
+
+    this.button.addEventListener('click', (e) => {
+      e.preventDefault()
+
+      this.button.disabled = true
+
+      this.submit()
+    })
+  }
+
+  async submit () {
+    const blob = await cropCanvasAndExportToPNG(this.canvas)
+    const file = new File([blob], 'signature.png', { type: 'image/png' })
+
+    const dataTransfer = new DataTransfer()
+
+    dataTransfer.items.add(file)
+
+    this.input.files = dataTransfer.files
+
+    if (this.input.webkitEntries.length) {
+      this.input.dataset.file = `${dataTransfer.files[0].name}`
+    }
+
+    this.closest('form').requestSubmit()
+  }
+})