The Serial Debuggers breakpoints are added at the end of the line in the temp copy of the code before upload.

If you move the breakpoint down to the { brace it will trigger when the if condition is met, or move it to the line above (378) where the dur variable can still be inspected.

Alternatively the GDBStub for the Uno may be an alternative option, which does not work by adding code in:
https://www.visualmicro.com/page/AVR-Debugging.aspx

It's probably worth reading the serial debugger guides in our docs. Everything I have said is clearly shown. You don't appear to have followed my comments in my previous response. 

Move breakpoint at 376 to 378 which is a simpler code line. 

other things to bear in mind

Bear in mind a fast cpu loop() can work faster than the pc in terms of rendering data to serial monitors and debug windows. You are right to try to debug inside conditions, that will help to keep the number of debug packets down. The breakpoints also provide conditions and the counter is treated as millis()

Your other post here https://www.visualmicro.com/forums/YaBB.pl?num=1688667266/2#2