Eclipse training, like we have this week in Poland, it's always great opportunity to learn new things, as students have lots of stimulating questions, that you'd never ask yourself :)
So today morning we were debugging JUnit tests and got caught by "Why Drop To Frame is inactive?".
Luckly, Darin Wright already answered to this question some time ago on bugzilla:
"the Java debugger supports the operation when
debugging on a VM that supports it (1.4.x or higher), and there are no native
methods on the call stack above the point you want to drop to."
That was the hit, as we had JUnit native methods on the stack.