Nested groups, one fixed to bottom, only allowed to sort above it

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Been using nested Sortable since 2019 and it's worked really well! Perhaps only problem is now having about 2800+ items, it gets a little slow (never figured out how to lazyload items without it breaking things.. mentioned a few times in issues, any example of best practice? forceFallback: true, seems to help) – but my main issue, is using nested Sortable for folders/filers, where one group is read-only demos. They should always be at the bottom of the nested group – just fixed a bug that allowed one to put into folders of that locked group, but now, if one tries to drop into the demos group, the file seems to disappear, however it's been placed below this locked group... is there a way to force/catch any attempt to drag something below a given group? Guessing it's about catching it during one of the events prior to store:? Thanks if anyone has solved this issue.

Edit: Maybe should also add that when modifying the demos, I unlock and allow dragging within this locked group, so I can export new ones – but 99% of the time, it's locked. So I don't want to completely isolate it.

FYI, using Sortable v1.9.0 (just tried upgrading, but caused breaking changes of multiple functions and couldn't find a guide for what to adjust)