Minimal test case in sketch TestBug:

TestBug:
void setup() {}
void draw() {}

Bar:
class Bar {}

Foo:
interface Foo {
Bar getBar();
}

Results of trying to run this example:
[...]/Temp/build7277.tmp/Temporary_2750_1563.java:5:10:5:12: Semantic
Error: The static type "Temporary_2750_1563$Foo" must use a qualified name
to access the non-static member type "Temporary_2750_1563$Bar" of the
enclosing type "Temporary_2750_1563".

Running Processing 0135 Beta under Windows.

If you're going to do interfaces, etc., just add .java extensions to the
filenames of those separate tabs so that they aren't confusing the
preprocessor. You don't want inner classes that are interfaces anyway.