Steps to Reproduce

run the attachment frozen-issues.zul
1. scroll the grid to the right
2. scroll to the page bottom
3. scroll the page back up (using the mouse wheel) quickly so that the mouse cursor is above the grid

updated simpler step sequence:
1. scroll to right
2. hover the grid
3. mousewheel down once (the real down not he macOS inverted direction)

Current Result

the scroll positions become inconsistent
the vertical grid scroll bar stops working
cells render above the frozen columns

using the horizontal scroll bar fixes the issues