I'm using Fiddler v4.6.20173.38786 (Built: Friday, September 15, 2017). When I click on the Customize Rules menu entry there is a message box that says "Unable to load Fiddler for code-completion support". After I click OK to that the Fiddler script editor will open normally.
There is no registry key InstallerSettings after installing from Administrator in HKCU\SOFTWARE\Microsoft\Fiddler2
Everything described above is still an issue in build Dec-14-17 v5.0.20173.50948 installer which is the latest version.
And code completion doesn’t work.