I tried both TCL/TK 8.5.13 and TCL/TK 8.6 with the latest Python 3.4 on Windows 7 the editor window never showed a line as I stepped through the debugger. I am going to try in Mac/Linux to make sure I am not crazy that a line in the editor window does indicate where the debugger is. Then I intend to look in the bug tracker on TCL/TK and possibly post a question for help. On Windows the initial line does look funny on a line with a quoted string such as print("hello world") the yellow does not cover the quoted string.