Slitherine

I can no longer access the sidebar (template that covers terrain to zones categories). It disappeared; even after uninstalling and reinstalling PzC and its add-ons. Is there some toggle button I might have inadvertently pressed? This is baffling.
Is it some idiosyncrasy with the program, my operating system (XP), or something else? It worked fine before. BTW, the palette filter selection is no longer available (highlighted), or is that a clue to the problem?

How comfort you're with editing Windows registry? Have you ever done it?
The solution to your issue, might be here:
HKEY_CURRENT_USER\Software\Slitherine\Panzer Corps\MapEditor

However, I'm not familiar with the editor, maybe there's a easier solution. I'll ask El_Condoro or Kerensky to look at your issue.
They live in a different time zone. So hang-on a couple of hours.

The only thing I can think of is that you have dragged the divider bar to the left so that it looks like part of the left border. If you hover your mouseover the left hand side you might be able to drag it back again. The palette filter is only highlighted when you have chosen Edit, Units. Let us know how you go.

I had something similar a couple of times, usually after minimizing/maximizing the editor window. After minimizing/maximizing again or restarting it's usually gone. Sometimes the divider remains on the left edge then, but can be dragged to the right again.

You were absolutely correct. It was hidden on the edge (left border of the screen). I dragged it back after hovering the mouse cursor at the edge of the frame. Thanks so much. Now I can get back to creating scenarios again.