TDrawGrid OnDrawCell clipping

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I have a Delphi program that uses TDrawGrid. I use a method of combining cells. If I want to combine, say. two adjacent cells A and B  in the same row, then in the OnDrawCell for cell B I do nothing. For cell A I get the cell rectangle and then calculate a new rectangle that covers both A and B, and write text and background to this extended cell. This works well, for things like multiple heading rows where a top heading extends across several sub-headings on the row below. It's also possible to selectively erase the grid lines by extending the cell rectangle just enough to cover the lines.

Using the same code in Lazarus does not quite work.  What happens is the extended cell rectangle gets clipped back to the original cell rectangle.

Is there a way of stopping Lazarus clipping in this way?

I can live without a solution to this, and goFixedHorzLine=false helps a little, but it's not as elegant.