page up and page down don't work as expected

Steps to Reproduce

1. load a new book with a blank sheet (maxVisibleRows="200")
2. click A1
3. press "page down" key, it scrolls down
4. press "page down" key again,

Current Result

keikai scrolls back to A1

Expected Result

keikai should scroll down again

Debug Information

1. "page up" key doesn't scroll up a sheet.