How to enable text overflow for merged cells in Calc?

I have merged cells in Calc. I would like the text to overflow into the adjacent, empty cells. However, even though I have “text overflow” enabled, the merged cell have the little red arrow (indicating that the text is larger than the cell) rather than the text overflowing into the adjacent empty cells.

Is text overflow for merged cells not possible?

Perhaps I misunderstood your question. I think you mean this kind of cell formatting (Ctrl+1)
Wrap text in cell