When setting a grid column with fixed cols and header it will set a high z-index which will overlap when adding floatable windows or other content in the DOM object.
Temporarily fixed by doing the following.
.dhx_header-fixed-cols {
z-index: 100 !important;
}
This issue did not exist prior to latest release 6.5.8
As of the latest release the z-index is set to 999999.