Created attachment 89612 [details]
screenshot

What number of pass is it? First or second?

first pass, I never expected more passes. 

code:
var a = 1;
var b = 2;
var c = a + b;

I step through the lines a, b. Stopped at c and expected results of a, b to be shown in Variables window. But they shows
after evaluation of the expression c. It behaves in different way then Java debugger

Fixed.

2792:3af259814088

I do not see the variable values in the Variables window at the first pass in the build 200910310201

Created attachment 90410 [details]
IDE screenshot

I suspect, this is basically problem of JavaFX bytecode. Try to step over several times, the are line number generated
for these lines which are not related to these expressions, later on this gets evaluated. IMHO, the debugger shows
correct values for incorrect bytecode.

Created attachment 90411 [details]
Screenshot (variables after being evaluated)

This seems to be Java Debugger issue.

With JavaFX debugger you must be patient. It does not work like Java debugger!

Consider following program:

18: var a = 10;
19: var b = 20;
20: var c = a + b;

1) put breakpoint to line 18 and start debugger
2) Program stops on line 18, Variables show a = 0, b = 0, c = 0
   Do not ask me why.
   These variables are simply already defined with initial values.
3) Step Over goes to line 19, but does not change variable values.
4) Step Over goes to line 20, but does not change variable values.
5) Step Over goes to package declaration (hmmmm)
6) Step Over goes to line 18 again!
7) Step Over stays on line 18, but something is already going to happen...
   Variables show "varNewValue$" as 10.
8) Step Over stays on line 18, but "a" is 10! Wow.
9) Step Over goes to line 19
10) Step Over goes to line 20 and "b" changes to 20!
11) Step Over goes to line 18 again! (hmmmm)
12) Step Over goes to line 20
13) Step Over and finally "c" changes to 30!

So the conclusion is: debugger works! :-)

First time the debugger shows zero values for the variables.

This is true, but this can not be fixed.
Sorry, it's how JavaFX works.
Please complain to JavaFX compiler, not debugger. And JavaFX compiler is out of control of NetBeans.

Yes, Alexander, look what is in classfiles - the behavior does exactly what classfile contain and we can not do anything for it. There is first pass when 0 values are assigned and this code has correct line numbers. During second pass correct values are updated. This is purely SDK/compiler issue.

verified as will not fix