Using Shift+F8 when stepping line by line when on a line of code that contains a SUB/FUNCTION call, the sub-procedure lines will be run without entering the block of code, and execution will resume in the next line of code after the SUB/FUNCTION call.
F9 (or clicking a line number) will toggle a breakpoint; When running a
program, F8 will pause/step line by line, F9 will still be usable to
toggle breakpoints and F5 will continue execution. ESC exits $DEBUG
mode.
