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[charles_piano]

It creates actually two system breaks (one on the current system and another on the next one), in order to lock the system you’re working on.

Note Spacing Tool adds system breaks

The assumption is that if you’ve got the level of tweaking note spacing, you want that system to be kept together (as obviously if the casting off subsequently changed, your tweaks would be in a different context and also in previous versions would have been lost entirely).

If you want to adjust the spacing without locking the system, you can insert a Note Spacing Change instead.