Not able to run Debugger

pgAdmin now up to 4.6
Postgres 9.6
Windows 7 Pro

I had trouble getting the debugger running about 6 weeks ago, posted a query, tried the suggestions, got no joy, got distracted by other tasks. Am now back at it.

I have deleted and recreated the debugger extension.
I have deleted and reinstalled Postgres itself. (EDB Windows binary installer)

C:\Program Files\PostgreSQL\9.6\lib contains plugin_debugger.dll

postgresql.conf contains :

shared_preload_libraries = '$libdir\plugin_debugger.dll'

When I try to set a breakpoint I get :

"The debugger plugin is not enabled. Please add the plugin to the shared_preload_libraries setting in the postgresql.conf file and restart the database server for indirect debugging."

The pgAdmin I am running is in C:\Program Files (x86)\pgAdmin 4\v4\runtime, however, EDB Postgres also installed an older version of pgAdmin4.exe (4.2) in C:\Program Files\PostgreSQL\9.6\pgAdmin 4\bin

Going to a command line, pg_ctl start complains

"$libdirplugin_debugger.dll": No such file or directory"

and shuts down Postgres. Yes, the missing "\" is exactly what is reported but NOT what is in the conf file.

If I hard code the path to plugin_debugger.dll it complains about that. I am not finding where $libdir is defined. I suspect the pgAdmin4 I am running is making assumptions that don't match the actual installed libdir path, and that moving that dll to the right place will solve the problem.

pg_config --libdir reports:

C:/PROGRA~1/POSTGR~1/9.6/lib

which is where plugin_debugger.dll is located (assuming the forward slashes, 8.3, etc. are translated correctly)

I tried setting

dynamic_library_path = 'C:\Program Files\PostgreSQL\9.6\lib'

in the conf file, but postgres gave the same error message.

Thanks in advance.