Strange Zoom feature in diagram

I tried to use the Show Diagram feature on a package in the project view.
Sometimes when I middle-click somewhere in the diagram IDEA shows something similar to a magnifying class:
How do I get rid of it again?
Why can't I invoke it consistently?
Or isn't this IDEA at all, but something else alltogether? (Though it doesn't seem to be some 3rd party accessibility tool.)

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