Since I'm sprucing up the bathroom, I figured I might as well get a nice new
supply line for the toilet.

Remember when a half gallon of ice cream suddenly shrunk to 1.75 qts and even 1.5 qts for some brands? Apparently the plumbing industry took notice.

Here's my old 12" supply line compared to the new 12" supply line.

https://i.imgur.com/QYEh8I9.jpg

Guess how much more supply line I need to tighten up the connection?