ui.libraryPanel.locate checked only that the panel was open. Run on an
open panel whose library was there, such as the built-in Pastels, it
loaded the file given or picked and showed that library in the panel
in place of Pastels.
It now answers "library panel `{given}` isn't missing its library" for
such a panel, before it asks for a file or loads one, and the panel
keeps its library. A persistent panel whose library's file is gone
locates as before.
Docs: docs/mcp.md (ui.libraryPanel.locate).
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