- Edited
OK @nrudnyk, think I have a revelation, thanks to @Irix 's comment earlier.
This issue APPEARS to happen only when there are >=2 screens connected.
The M4 Mini has 2 screens (2x 2560x1440 27" 144 Hz displays; one by HDMI, and one by Thunderbolt to Displayport).
I unplugged the HDMI display out of the M4 Mini, and can now successfully drag/drop tabs without beachball on the Mini. That is a change from before, showing that with only 1 screen, there's no beachball freeze on the M4 Mini.
I then plugged that HDMI cable into my M4 MBP, and immediately could get the beachball freeze to happen on RC on the laptop. That shows the issue doesn't happen until there's > 1 screen, then it will.
The moment I unplugged the second screen (no need to restart Orion), I can then drag a window without issue.
Thoughts - I wonder if display scaling is relevant here? These are not HiDPI screens. They are also high refresh rate (144 Hz). But I think this is the missing reproduction step!