I tried resetting to default keybindings. Now when I right-click, I get the menu on the click-down, without releasing. I drag with the button held down, and my cursor moves over the menu, rather than selecting.
When I switch keyboard emulation from CUA to "SlickEdit (text mode edition)", the behavior is the same. When I switch to vim, it is the same as well.
BUT, when I start slickedit with a clean config using the command you suggested (vs -sc path), it just works. The right-click select works the way I expect it to. Thanks for your help--this gives me a place to start to try to figure out what's wrong.