Is it really a bug? The easy thing to do is to resize the window by grabbing anywhere along the right hand edge! All additional space is given to the preview. That seems logical and intuitive to me.

You can also double click on the last of the column separators.

Resizing windows only works on files with short paths. Double-clicking column separators only works when you're reducing the width of the second-to-last column and I still had to adjust the height of the window for it to max-out the preview of my .txt document I tested. If the intent is to allow every column to be resizable, and the assumption is that for the last column, you can simply resize the window, then yes, this is a bug - because that doesn't work for files with long paths. If the developers knew that eliminating the last column separator would prevent resizing of columns for files with long paths, then it's not a bug, but rather a poor design decision.