[Bug Report] transform-origin在页面有滚动条时，出现边框线消失

### VUX version
2.9.1-rc.1

### OS/Browsers version
win10/chrome 65.0.3325

### Vue version
2.5.16

### Code

```html

import {
XButton,
Group,
Cell,
CellBox,
Card,
Radio,
XInput,
Selector,
PopupPicker,
Datetime,
XNumber,
Calendar,
XAddress,
XTextarea,
XSwitch,
dateFormat
} from 'vux'

const NOW_TIME = dateFormat(new Date(), 'YYYY-MM-DD');
export default {
name: "newLeaveOd",
components: {
XButton,
Group,
Cell,
CellBox,
Card,
Radio,
XInput,
Selector,
PopupPicker,
Datetime,
XNumber,
Calendar,
XAddress,
XTextarea,
XSwitch
},
data() {
return {
odTypeOpt: [[
{
name: '事假',
value: '1'
},
{
name: '病假',
value: '2'
}
]],
startTimeOpt: [[
]],
endTimeOpt: [[
]],
leaveSendForm: {
type: ['1'],
tiaoxiuTime:
{
restStartDate: NOW_TIME,
restStartTime: ['08时20分'],
restEndDate: NOW_TIME,
restEndTime: ['18时30分'],
workStartDate: NOW_TIME,
workStartTime: ['08时20分'],
workEndDate: NOW_TIME,
workEndTime: ['18时30分']
}
}
}
}
}

```

### Steps to reproduce
注释掉代码中的除group2以外的group，结束日期选择项上边框线出现。解开注释，上边框线消失，其中变化是出现滚动条。
为方便查看，删减了不必要的很多代码。注释掉组件（vux-calendar）的transform-origin: 0 0属性，其中一个恢复正常，继续注释掉transform: scaleY(0.5)，页面中所有边框线出现。暂时不明白中间的缘由，可能是为了1px的边框？

### What is Expected?
出现滚动条时页面边框线不应该消失

### What is actually happening?
边框线消失

Start with the vux-calendar component and inspect its transform-origin: 0 0 and transform: scaleY(0.5) styles. Reproduce the page with and without the extra groups so a scrollbar appears, then verify that the border above the end-date selector remains visible without breaking the intended 1px-border rendering.

Scritto dal modello di indicizzazione a partire dal testo della issue.