Steps to Reproduce

https://zkfiddle.org/sample/2itfi07/1-toggle-maximize-button-cause-line-break

click the maximize button

Current Result

Expected Result

no line break and no width change before/after toggle maximize button

Debug Information

https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/wnd/Panel.js#L354

https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/wnd/Panel.js#L645

clientWidth cause some float deviation
try using getBoundingClientRect()
also check Window

Workaround