While progress is coming along great, I don't feel the Jumper will truly be ready on time, to a point of "no further changes that might have significant impacts on levels will happen", if I try to stick to the normal release schedule. I also haven't got around to making too much progress on most of the other things I want to get done for this update - projection shadow is done, but aside from that, all that's done is a few minor bugfixes.
I also don't think that the above is to enough of an extent to justify "the Jumper as a whole needs to be delayed until V12.11.X", so instead, I'm going to slightly delay V12.9. Most likely, the RC will drop towards the end of this month or the start of next month, and as usual, the stable build about a month after that. Whether this becomes a permanent delay (ie: the release schedule in general is pushed back a month) or a one-off (ie: V12.10.X still drops at the time it would have were V12.9.X not delayed) will be decided later, and will likely come down to whether I feel I can get V12.10's features done in a shorter time (which might be feasible, as there aren't really any particularly major features planned for it at this stage).
In the meantime of course, the Jumper experimental build (https://www.lemmingsforums.net/index.php?topic=4718.0) is available, which can be used to help bug-test, or to start planning your levels (though this should be done with the understanding that later updates to Jumper physics may require tweaking your levels - we're mostly dealing with edge cases at this point, not the main functionality, but be prepared for it just in case).