Make flexibleBox visible position cadidate only where it is editable.

LayoutTests/fast/events/drag-in-flex.html

+<style>
+div {
+  display: flex;
+  font-family: 'Helvetica Neue', sans-serif;
+  zoom: 150%;
+}
+
+</style>
+<div>
+  <span>Drag from behind of button to this text.</span>
+    <button></button>
+</div>
+<script>
+function applyZoom(rect, zoom)
+{
+  var newRect = {
+    left : rect.left * zoom,
+    top : rect.top * zoom,
+    width : rect.width * zoom,
+    height : rect.height * zoom
+  };
+  return newRect;
+};
+
+var zoom = getComputedStyle(document.querySelector('div')).getPropertyValue('zoom');
+var buttonRect = applyZoom(document.querySelector('button').getBoundingClientRect(), zoom);
+var spanRect = applyZoom(document.querySelector('span').getBoundingClientRect(), zoom);
+
+if (window.testRunner) {
+  eventSender.mouseMoveTo(buttonRect.left + buttonRect.width * 2, buttonRect.top + buttonRect.height / 2);
+  eventSender.mouseDown();
+  eventSender.leapForward(100);
+  eventSender.mouseMoveTo(spanRect.left + spanRect.width / 2, spanRect.top + spanRect.height / 2);
+  eventSender.mouseUp();
+}
+</script>