An update on this since the 5.1 upgrade. I’m not sure if anything has changed in the code, but the way this problem manifests has changed. instead of the choose object popup flashing up but disappearing straight away, now it doesn’t appear at all. nothing happens. two overlapping curves, and it behaves as if they are both on a locked layer - no choose object popup at all. however, if i then draw a line elsewhere and come back, it seems to resolve, and the choose object popup appears. this may be a new issue, or a mutation of the original one, im not sure - but the original problem hasnt happened since I updated.
the more i use the 5.1 upgrade, the more this seems to be becoming a constant annoyance. maybe its as my model is becoming more complex, but it is jamming up on this quite frequently now. every few minutes. it seems to jam up other things as well - some of the menus for eg, and occasionally it does this weird thing with selections, where I type in (for example) Scale1d, but it rather than letting me then select an origin point, it just selects something at random, possibly whatever was under the cursor - im not 100% sure. but Rhino has become a whole lot flakier with the 5.1 update, thats for sure.