Hi Manni,

    The grid tooltips are the same size as the cell. I understand that something this can look a little odd in a merged cell case where the text is small. But the grid can't really make any intelligent judgements about when this looks good and when it doesn't.

    You can alter this behavior using a CreationFilter.

    Here's a sample CreationFilter that displays normal tooltips on Merged Cells. You just copy this code and set the grid's CreationFilter property to an instance of the MergedCellToolTipCreationFilter class. 

    #region MergedCellToolTipCreationFilter
    public class MergedCellToolTipCreationFilter : IUIElementCreationFilter
    {
        #region IUIElementCreationFilter Members

        void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
        {
            // Watched for MergedCellUIElements
            if (parent is MergedCellUIElement)
            {
                // See if it has a tooltip item and that it is not already a custom
                // MergeCellToolTipItem
                if (parent.ToolTipItem != null &&
                    (parent.ToolTipItem is MergeCellToolTipItem) == false)
                {
                    // Replace the existing TooltipItem with a custom MergeCellToolTipItem
                    parent.ToolTipItem = new MergeCellToolTipItem(parent.ToolTipItem);
                }
            }
        }

        bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
        {
            // Do nothing, just return false.
            return false;
        }

        #endregion
    }
    #endregion //MergedCellToolTipCreationFilter

    #region MergeCellToolTipItem
    public class MergeCellToolTipItem : IToolTipItem
    {
        private IToolTipItem baseToolTipItem;

        public MergeCellToolTipItem(IToolTipItem baseToolTipItem)
        {
            this.baseToolTipItem = baseToolTipItem;
        }

        #region IToolTipItem Members

        public ToolTipInfo GetToolTipInfo(Point mousePosition, UIElement element, UIElement previousToolTipElement, ToolTipInfo toolTipInfoDefault)
        {
            // Create a new ToolTipInfo based on the values passed in.
            ToolTipInfo newToolTipInfo = baseToolTipItem.GetToolTipInfo(mousePosition, element, previousToolTipElement, toolTipInfoDefault);           

            // Set the location to the location of the mouse.
            newToolTipInfo.Location = mousePosition;

            // Set the size to empty so the tooltip will automatically calculate the size based on the
            // contents.
            newToolTipInfo.Size = Size.Empty;

            // Exclude a rect where the mouse point is.
            Rectangle rect = new Rectangle(mousePosition, new Size(0,20));           
            newToolTipInfo.ExclusionRect = rect;

            // Return the new tooltip info.
            return newToolTipInfo;
        }

        #endregion
    }
    #endregion //MergeCellToolTipItem