I created a simple page of WebGrid based on your description and managed to reproduce the reported problem in my local test.

Adding the following JS code fix the edit box misplaced problem in my local end. Thanks to Leo Chandra for providing the script.

function EnterEditMode(controlId, tblName, editObject)
{
    if (IS.chrome || IS.moz)
    {
        var grid = ISGetObject(controlId)
        var element = editObject.element;

        if (editObject.element.GetFrameElement)
            element = editObject.element.GetFrameElement();

        var offsetLeft = ISPosLib.getLeftNonIE(grid.FrameObj) - ISPosLib.getLeft(grid.FrameObj);
        element.style.left = (parseInt(element.style.left) + (offsetLeft)) + "px";
    }
}

Try to assign the above script to WebGrid's OnEnterEditMode client-side event. Please feel free to let me know if you have different result.

I've added the code you proposed and it works great now. 

Many thanks!

Glad to hear that the snippet code helps to resolve the problem - WebGrid edit box misplaced when browser's window is scrolled.

Should you need further assistance or run into any problems regarding our controls, feel free to post it into our forum. We would be happy to assist you again.