I just tested again and found that it happens when you place the grid inside another control. WebPageControl in my case.
When I put in directly on the form it works fine.
Regarding the CSS : i have a css from you to fix some WebDGGrid issues( post: Unexpected look WebDbGrid)
I am on v1.5.6.0
Can that be the reason :
/*
We saw the issue. We'll fix this.
Workaround, add following CSS:
<style type="text/css" id="WebStringGrid1_style">table td { overflow: hidden; text-overflow: ellipsis; white-space:nowrap; border: solid 1px #cccccc; padding: 0px;font-family:Arial;font-style: normal;font-size:8pt;} table td.selected { background-color: #5B8FFF; color: #FFFFFF; } table tr.selected { background-color: #5B8FFF; color: #FFFFFF; } </style>
*/