Ok, I have an NSTableView in my window in Interface Builder and I’d like it to be at a constant width of 250 (even when the user resizes the window !!!). I’d also want it to stay on the right side, like a side panel or so. I also want the cells to stay at the exact same width.
I have added an image below. I guess there are some options in Interface Builder right ?

You should go to the metrics pane in Interface Builder and set your table view autosizing options like in the picture below
In this way, your table view will stick to the right side and not grow (i.e., will keep its width).