Surface event for file-attachment-dragged

The implementation for github/github#190219 needs
an event to kick off finding the cursor position, and rather than bubbling the
DragEvent it seems prudent to fire a CustomEvent solely used by that consumer.
That'll avoid any unintended side effects in the document at large.

Author: Bestra

src/file-attachment-element.ts

@@ -73,6 +73,17 @@ function onDragenter(event: DragEvent) {
   if (dragging) {
     clearTimeout(dragging)
   }
+
+  const dragEvent = new CustomEvent('file-attachment-dragged', {
+    bubbles: true,
+    cancelable: true,
+    detail: {
+      dragEvent: event,
+      attachmentElement: target
+    }
+  })
+  target.dispatchEvent(dragEvent)
+
   dragging = window.setTimeout(() => target.removeAttribute('hover'), 200)
 
   const transfer = event.dataTransfer

test/test.js

@@ -124,6 +124,24 @@ describe('file-attachment', function () {
       assert.equal('test.png', event.detail.attachments[0].file.name)
       assert.equal(0, input.files.length)
     })
+
+    it('fires a file-attachment-dragged event on dragenter', async function () {
+      const listener = once('file-attachment-dragged')
+      const dragEvent = new Event('dragenter', {bubbles: true})
+      input.dispatchEvent(dragEvent)
+      const event = await listener
+      assert.equal(dragEvent, event.detail.dragEvent)
+      assert.equal(fileAttachment, event.detail.attachmentElement)
+    })
+
+    it('fires a file-attachment-dragged event on dragover', async function () {
+      const listener = once('file-attachment-dragged')
+      const dragEvent = new Event('dragover', {bubbles: true})
+      input.dispatchEvent(dragEvent)
+      const event = await listener
+      assert.equal(dragEvent, event.detail.dragEvent)
+      assert.equal(fileAttachment, event.detail.attachmentElement)
+    })
   })
 })