Steps:
Currently, what happens is that block A is moved below block B:

Testing 3.9.-RC ...
The drag & drop seems to be intermittent. It’s hard to nail down, though, what exactly goes wrong when.
Sometimes the landing zone is on the top of another block, but when I release the block it actually lands underneath the block.
The scrolling is delayed so when I try to move things below the top of the bottom edge it doesn’t seem to wanna go….
It feels a bit arbitrary – here is a screen recording of such a session: http://recordit.co/AQYO4qrr1R
Google Chrome V. 68.0.3440.106
OS MAC 10.13.6
Thanks for the report and the video. As a follow-up to the slack convo, there are several issues at play on that video that we are already tracking such as scrolling, consecutive drop zones, etc. We're reviewing the _drop experience_ and hope to ship some improvements over the next few versions.
This will be fixed by https://github.com/WordPress/gutenberg/pull/10201 which is scheduled for 4.0.
https://github.com/WordPress/gutenberg/pull/10201 has been merged and the fix will be included in the 4.0 RC.
Most helpful comment
Thanks for the report and the video. As a follow-up to the slack convo, there are several issues at play on that video that we are already tracking such as scrolling, consecutive drop zones, etc. We're reviewing the _drop experience_ and hope to ship some improvements over the next few versions.