steps to reproduce

1. open dev tool /network tab
2. load the zul with the attached file

   <zk>
   	<spreadsheet id="ss2" src="/WEB-INF/excel/preload.xlsx" showToolbar="true" maxVisibleRows="100" maxVisibleColumns="100" width="700px"
   		height="500px" preloadColumnSize="100" preloadRowSize="100"/>	
   </zk>
   

3. constantly move selection box to the right with right arrow key to column 22

result

onZSSCellFetch is sent when moving to column 22, then every 2 columns move by the right arrow key triggers an onZSSCellFetch being sent.
Moving down to row 22 also triggers the event, then every 2 rows move by the key triggers an onZSSCellFetch being sent.

expected result

At the beginning,
dtid:z_1pg
cmd_0:onZSSSyncBlock
uuid_0:b99P0
data_0:

{"sheetId":"b99P0","dpWidth":8800,"dpHeight":2000,"viewWidth":683,"viewHeight":419,"blockLeft":0,"blockTop":0,"blockRight":20,"blockBottom":20,"fetchLeft":-1,"fetchTop":-1,"fetchWidth":-1,"fetchHeight":-1,"rangeLeft":0,"rangeTop":0,"rangeRight":99,"rangeBottom":99}

the blockRight is 20 (21th column), with preloadColumnSize, zss should get 20+ (100/2) = 70 coulumns.
It supposes to move more than 70 columns, then ZSS should fetch the data from the server.