Check that stepInto at then end of the script go to next user script instead InjectedScriptSource.js.
paused at:
(function boo() { setTimeout(() => 239, 0); #debugger; })()


paused at:
(function boo() { setTimeout(() => 239, 0); debugger; #})()


paused at:
(function boo() { setTimeout(() => 239, 0); debugger; })()#


paused at:
(function boo() { setTimeout(() => #239, 0); debugger; })()