updated sclass on combobox doesn't update popup

Steps to Reproduce

http://zkfiddle.org/sample/26va68c/1-sclass-on-combopopup
1) open the popup
2) click the button

Current Result

the sclass is only removed on the combobox itself, the combobox popup keeps the sclass

Expected Result

the sclass on both the combobox and the popup should be synchronized

Debug Info

Root Cause

Workaround

call invalidate after updating the sclass.