Steps to reproduce

1. load attached excel file

   	<spreadsheet id="ss" width="100%"  height="100%" showSheetbar="true" hidecolumnhead="true"
   		maxVisibleRows="100" maxVisibleColumns="50"  maxRenderedCellSize="5000"
   		src="/WEB-INF/issue/t1979.xlsx"/>
   

2. scroll to right quickly

Actual result

Uncaught TypeError: Cannot read property 'i' of undefined
zss.Header.zk.$extends.$init @ zss.wpd:19
init @ zk.wpd:9858zss.Panel.zk.$extends.createHeaders_ @ zss.wpd:27
zss.TopPanel.zk.$extends.create_ @ zss.wpd:29
zss.MainBlockCtrl.zk.$extends.create_ @ zss.wpd:24
zss.MainBlockCtrl.zk.$extends._loadForVisible @ zss.wpd:24
zss.MainBlockCtrl.zk.$extends.loadForVisible @ zss.wpd:24
(anonymous function) @ zss.wpd:24
zss.Spreadsheet.zk.$extends.onResponse @ zss.wpd:2_invoke @ zk.wpd:12614
zk.$extends.fire @ zk.wpd:12597
(anonymous function) @ zk.wpd:12735
setTimeout (async)_fire @ zk.wpd:12738
zWatch.fire @ zk.wpd:12831
zk.override.fire @ zk.wpd:14137
doCmdsNow @ zk.wpd:19545
zAu._doCmds @ zk.wpd:19780
afterResponse @ zk.wpd:19424
onResponseReady @ zk.wpd:19421
XMLHttpRequest.send (async)ajaxSendNow @ zk.wpd:19484
zAu.sendNow @ zk.wpd:19938
(anonymous function) @ zk.wpd:19463
setTimeout (async)ajaxSend2 @ zk.wpd:19463
ajaxSend @ zk.wpd:19457
zAu.send @ zk.wpd:19698
zk.Widget.zk.$extends.sendAU_ @ zk.wpd:17069
zk.Object.$supers @ zk.wpd:10655
zss.Spreadsheet.zk.$extends.sendAU_ @ zss.wpd:2
zk.Widget.zk.$extends.fireX @ zk.wpd:17048
zk.Widget.zk.$extends.fire @ zk.wpd:17077
zss.MainBlockCtrl.zk.$extends._sendOnCellFetch @ zss.wpd:24
zss.MainBlockCtrl.zk.$extends.loadCell @ zss.wpd:24
zss.MainBlockCtrl.zk.$extends.doScroll @ zss.wpd:24
zss.ScrollPanel.zk.$extends._doScroll @ zss.wpd:35
zss.ScrollPanel.zk.$extends._doHScroll @ zss.wpd:35
(anonymous function) @ zss.wpd:35
setTimeout (async)zss.ScrollPanel.zk.$extends._fireOnHScroll @ zss.wpd:35
zss.ScrollPanel.zk.$extends._trackScrolling @ zss.wpd:35
zss.ScrollPanel.zk.$extends._doScrolling @ zss.wpd:35
zk.Class @ zk.wpd:10582
p @ zk.wpd:14767
jQuery.event.dispatch @ zk.wpd:5162jQuery.event.add.elemData.handle @ zk.wpd:4833

Debug information

• scroll slowly doesn't have this issue
• hidecolumnhead="false" doesn't have this issue