My pde seems to have got itself into a state where it can only use classes
defined in the 'primary' tab; by which I mean the tab with the sketch's name.

example 1:
Primary tab 'BClass' tab
------------ -------------
class Aclass { class Bclass {
} }
Aclass A;

Bclass B;

When I hit the 'run' button I get 'Cannot find a class or type named "Bclass" '

But...
Example 2:
Primary tab 'BClass' tab
------------ -------------
class Aclass { class Bclass {
} }
class Bclass {
}

Aclass A;

Bclass B;

Clicking 'run' produces "Duplicate nested type Bclass"

A little more investigation reveals that I have only one multi-tab sketch
that works properly. This was the sketch I was primarily working on when I
first experienced this problem in a second pde instance. For clarity,
everything worked fine previously.

The problem has survived reboot.

Sorry, I got too clever with formatting and produced something that is hard
to read. Let me clarify my examples and resubmit.

My pde seems to have got itself into a state where it can only use classes
defined in the 'primary' tab; by which I mean the tab with the sketch's name.

example 1:
Primary tab
------------
class Aclass {
}
Aclass A;

Bclass B;


'BClass' tab
-------------
class Bclass {
}

When I hit the 'run' button I get 'Cannot find a class or type named "Bclass" '

But...
Example 2:
Primary tab
------------ -
class Aclass {
}
class Bclass {
}

Aclass A;

Bclass B;

'BClass' tab
-------------
class Bclass {
}

Clicking 'run' produces "Duplicate nested type Bclass". Which is
inconsistent with example 1.

A little more investigation reveals that I have only one multi-tab sketch
that works properly. This was the sketch I was primarily working on when I
first experienced this problem in a second pde instance. For clarity,
everything worked fine previously.

The problem has survived reboot.

I substituted one problem for another when I tried to create a simple test
case. The problem with my example can be fixed with with void draw() {}.
Sorry for the red herring.

The only other clue I can offer is that while I was having my problem, task
manager showed a java.exe process persisting after all the pde windows were
closed. However, everything seems to be working fine following the reboot.

I've tried repeating my actions but I am unable to replicate the problem.

Add a draw() or setup() method. And keep separate classes in the other
tabs, or at the minimum, put setup/draw before your new classes. Otherwise
you're confusing the preprocessor into thinking that you're defining your
own PApplet class.