我正在尝试在自定义Polymer元素中包含paper-dropdown-menu元素，我看到以下错误：
Uncaught NotSupportedError: Failed to execute 'animate' on 'Element': Partial keyframes are not supported.
animation-constructor.js:61 Uncaught NotSupportedError: Failed to execute 'animate' on 'Element': Partial keyframes are not supported.
animation-constructor.js:61 Uncaught NotSupportedError: Failed to execute 'animate' on 'Element': Partial keyframes are not supported.
还有这个警告：
core-dropdown-overlay: dropdown's offsetParent must be the relatedTarget or the relatedTarget's offsetParent!
纸张下拉菜单显示正常，单击这些错误/警告时会发生这些错误/警告。嵌入页面本身时，代码工作正常。
我的网页结构如下：
<page-form>
...
<subform-element></subform-element>
...
</page-form>
页面表单是另一个表示我的页面的聚合物元素，子表单元素是一个包含纸张下拉菜单的表单。
<polymer-element name="subform-element" noscript>
<template>
<link rel="stylesheet" href="subform-element.css">
<div layout horizontal center>
<paper-dropdown-menu label="Disease" layered flex>
<paper-dropdown class="dropdown core-transition core-closed">
<core-menu class="menu">
<paper-item tabindex="0">Foulbrood</paper-item>
<paper-item tabindex="0">Varroa</paper-item>
<paper-item tabindex="0">Tracheal Mites</paper-item>
<paper-item tabindex="0">Other</paper-item>
</core-menu>
</paper-dropdown>
</paper-dropdown-menu>
<paper-input-decorator label="disease notes" floatinglabel="disease notes" flex>
<input is="core-input" placeholder="disease notes">
</paper-input-decorator>
</div>
</template>
</polymer-element>
我在Polymer版本0.5.5和Chrome 41中看到错误。