As reported here:
The grid view header height become taller by ifself when you scroll to the right.
There's also related issue reported before:
Both is cause by the poorly implemented Frozen feature on Zk CE. This is unlikely to be fixed by Zk as the much better working, so called "Smooth Scrolling Frozen" is a Zk EE only feature.
For now, we have to disable the use of Frozen until Zk improve the CE implementation or we re-implement their EE implementation. I've take a brieft look at their CE implementation - the key diffference is the EE approach is using "transform: translate3d(tx, ty, tz)" and "z-index:1" to reposition the Frozen column whereas the CE approach is by setting the width of the columns after the Frozen column to zero.