Are you using BeginUpdate/Endupdate?
Normally not, but I tested it (Linux Mint / Lazarus trunk / qt & gtk2) with using it and without - behaviour is the same.
We have investigated this here and have applied a fix for this issue, the next version will address this.
Thank you :-)
Additionaly it seems to be that the Columns.HorzAlignment is not working correctly for fixed cells/columns
You either need to use Columns, or cell properties. To use the HorzAlignment you can turn off the columns with Grid.UseColumns := False;
Ok, I guess I got it. At first, its a little bit confusing. UseColumns = false is by default, so when defining a column alignment and after that defining a different alignment for a single cell in that column, the alingment for the complete column changes. When I set UseColumns = true, I can redefine the alignment for just a single cell and the alignment for the rest of the columns cells are unchanged. Or with other words, when set UseColumns = false, the cell alignment is valid for the complete column.