It's not exactly variables. The Bugzilla entry is more specific: "... entails copying all the values that are currently on the interpreter's stack (arguments, local variables, intermediate results) from the heap onto the native stack." https://bugzilla.mozilla.org/show_bug.cgi?id=1839139#c8

So it may be a large function with lots of temporary values visible in scope. If I understand correctly, local lambdas would also count and they have their own captured context. I'm sure it's possible to find a pathological-but-not-unreasonable way to reproduce it.