Such a helpful thread! Thank you all for replying. Quick recap:
1. Mac can drag and drop b/w all apps, Win only Explorer, Linux NOTHING
2. This has been discussed in Linux for >20 years
3. It's apparently tied to a networking issue with X11?
4. It's possibly not in other DEs? (please let me know!)
5. It's now considered "fixed" and there is little interest in fixing it