I think it's Reaper's a bit clumsy way to tell you there are envelopes moving (as Eric assumed). I think I am with Eric's idea to (temporarily) show them on the media lane while they are getting moved. I'd file it under FR. Nice pic, btw :)
Thanks for replies guys :) I'm guessing the intent is to indicate envelope copying, so the fact that it interferes with the copying itself (or at least makes it inconvenient) in certain circumstances, I feel it should be filed as a bug.
Added in issue tracker. ---- Issue tracker related question to mods: When asked for affected version, it's sometimes not possible to know exactly since when a bug is in place, so should we just put the most recent version, or should the "Unknown" be also a viable choice (currently it's not possible to set as unknown). For this I put 3.0, as the quick search through history did not reveal any changes in control+drag/envelope copying.
Agreed :) It's an issue that users don't see often, but it's still an issue. People tend to find workarounds and forget about it, but it really should 'just work' eventually :) e