Thank you for your post.
I am unable to replicate the issue. This is what I have done.
1. Using FileMaker Pro 14.0.1 under Mac OS X 10.10.4, I created a database file with one Text field "Name".
2. I went into Script Workspace and created a New Script.
3. The first line of the script, I entered: If
4. I went into Specify Calculation and entered: 123 + 456
5. I moved my cursor to the beginning of the line, entered: // Comment and pressed Return. No change in font.
6. I created another New Script, entered: If
7. I went into Specify Calculation and entered: 123 + 456
8. I saved the script, edited the script, edited the calculation, put my cursor at the top, entered: // Comment and pressed Return. Still no change in font.
Let me know what I'm doing differently than you so I can replicate the issue: