https://help.libreoffice.org/Calc/Optimal_Column_Width
Double-click right column separator in column headers doesn’t work for me. I can size it manually, but double clicking doesn’t do anything. Further if I use Format/Column/Optimal Width, I get a 0.0 or 0.1 as default, but it only budges it 0.0 or 0.1. It will not resize it to the max size of the longest cell and then add 0.1 inches. Instead it will widen all cells by 0.1 inches.
If the column has wrap text enabled, then it won’t respond to an Optimal Width command no matter how you try to invoke it. Select the column, Format/Text/Wrap Text, (which is a toggle between “wrapped text” and “unwrapped text”.) If the contents of the column change to linear, and the next column to the right has filled cells, then the selected column will truncate its contents on the right hand border and be followed by a red triangle pointing to the right. In this condition (unwrapped), the optimal width command will operate.