sheet tab scroll buttons are at wrong position

Steps to Reproduce

1. visit the zul with Chrome or firefox

<spreadsheet id="ss" vflex="1"
		showToolbar="true"
		showSheetbar="true"
		showAddRow="false"
		showAddColumn="false"/>

2. click "New Book"
3. add many sheets until the left and right sheet tab scroll button appears
4. select the 2nd sheet

Other Steps

1. do the first 3 steps as above
2. shrink a browser width

Current Result

sheet tab scroll buttons move to a wrong position (almost out of the visible area)

Expected Result

they keep at the original position

Debug Information

• caused by showAddRow="false" and showAddColumn="false"

Workaround

.zstbtn-addRow, .zstbtn-addCol