Summary: Trackpad zooming and scrolling partly broken in last several versions.
Details: Trackpad zooming usually works fine, but sometimes fails and must be disabled-reenabled in Skim and only Skim . No other app has this issue. None. The problem is with Skim -- please don't tell me it's Apple, etc. (the usual response), because I don't have the problem with other PDF apps on the same system. There is no single, obvious trigger (a bad file, bug only appears after restart, with mulitiple files/tabs, etc.) It's random, or near enough.
System: Fully updated Skim and Mac (clean install in March, automatically updated since).
The fact is that trackpad zooming is implemented by Apple's PDFKit. We are
simply not involved.