thank you for your answer. Unfortunately HTML formatting is no solution for me.
Attached is a converted sample code to work with Telerik Winforms (zip renamed to .png). It works like this:
The content is taken from first the richtextbox outside the gridview. Adding / removing its text will cause all richtextboxes rendered in all grid cells to change, including rtb's vertical scrollbar. You have to move the mouse over the grid cell to make it refresh.
In an ImageColumn the winforms richtextbox is rendered without its WinForms scrollbars by CellPaint.
In order to fake the winforms richtextbox's vertical scrollbal, a Telerik radVScrollbar is rendered by CellPaint in the same cell.
When the active cell is selected, and the row size is reduced so much that the scroll thumb cannot fully render any more, so the thumb is hidden programmatically.
But for non active cells it will not work sometimes: upon row-height-change the vertical scroll thumb will not be hidden, but it will mess up the scroll arrow sometimes. This is probably due to the global vScrollbar and the sequence of the used events radGridView1_RowHeightChanged and _CellPaint and the wrong setting of the vScrollBar's size.
Anyhow I want to load the richtextboxes with unique content in each cell from a generic list of objects containing rtf string, thus different vertical scrollbar sizes are needed.
Also when resizing the column with the richtextbox, the width will be reduced, but after a while the vertical RadScrollbar starts to disappear.
I would appreciate changed code from you with fixes, I am sure there is a more clear approach.