I have simplified the problem to make it HOPEFULLY easy to re-produce.

Download the three files into a folder:

• StackOverflowProblem.R
• UpdatedShinyUIFunction.R
• OldShinyUIFunction.R

Edit [StackOverflowProblem.R]:

• Set a breakpoint on line 14, the call to renderPrint
• Run and you will see that the breakpoint is reached
• You will see “RR Testing” on the third tab “Cities by Name”
• Comment out line 9 source("OldShinyUIFunction.R")
• Un-comment line 10 source("UpdatedShinyUIFunction.R")
• Save and run – the breakpoint is not reached, and you will not see “RR Testing” on the third tab

I’ve used lintr, it was only finding trivial source code issues (mostly blanks)
I’ve used Notepad++ to count opening and closing brackets, and they all balance out.

I’m clearly missing something, but I don’t know what it is.

I am doing all the asking and none of the answering on SO, and I don’t feel very good about that.
If I can help you, let me know how.

Any pointers most appreciated!

Richard