Ned, I reread #20580.  It reminds me that both Control and Alt are problems.  In built-in config-extensions.def, the builtin fixed bindings and one of the configurable bindings are:

autocomplete: <Key-Tab> <KeyRelease-period> <KeyRelease-slash> <KeyRelease-backslash>
calltips: <KeyRelease-parenleft> <KeyRelease-parenright> <KeyRelease-0>
parenmatch: <KeyRelease-parenright> <KeyRelease-bracketright> <KeyRelease-braceright>
runscript: <Key-F5> (configurable)

I presume that these all work fine on Macs.  Every other configurable binding uses 'Control' or 'Alt'.  When I copy configurable bindings into the file, I have to put them in each section.  Should I change each to 'Command' or 'Option' for the 'Mac' and 'Osx' sections?  Also see comment on #20580.

I am adding #27173 as a dependency, for now, because it might be easier to include the new key set before starting to add new bindings.