How to auto fit columns in Finder when in Columns view? When in Finder Columns view, how can I automatically resize a column to autofit filenames?
You can do this by double-clicking on the border between columns. Note that doing this auto fits the column to the left.

In older versions of macOS (and/or if you have enabled always show scroll bars in System Preferences > General), you can do it by double-clicking the block below the scroll bar arrows.