I verified that inactiveselectbackground exists and starts as ''.  Since it should match the active selectbackground on all systems, as set by the current theme. I will try adding a line where selection colors are set: EditorWindow.ResetColorizer:
        self.text.config(
            ...
            selectforeground=select_colors['foreground'],
            selectbackground=select_colors['background'],
            inactiveselectbackground=select_colors['background']
            )
I believe this is the only place where this is needed.  The select colors are used for a different widget in TreeWidget. The use in ParenMatch for 'paren' tag should be temporary.

We will need to reconsider the 'found' highlight and the select versus found usage after this change.