I've just noticed that loading a library closes any open block editors. This is a problem, because one could lose their progress on working on a custom block, just by loading a library that they want to use.
It's a side effect of intentionally refreshing the IDE when loading a library. We need to do it in v10 in order for any customizations to palette primitives to take effect. I guess we could probably only refresh the IDE if a library does, indeed, customize the palette primitives. I've put this issue on my list. Thanks!