opening a menupopup moves a scrollbar of a long menu back to top

Steps to Reproduce

1. scroll down to hover on the last menu

Current Result

the scroll bar goes back to the top position.
submenu "last" doesn't open

Expected Result

the scroll bar keeps the current position and doesn't move.

Debug Information

• because .z-focus-a is always at the first child element (top) of the menupopup. When closing a sub menupopup and focus back to the parent menupupup's focus anchor. It also scrolls back to the top.
• Before 9.6.0.2 (included), there is no such a bug.
• caused by the fix of ZK-4598

workaround

zk-5235.js