OK, while testing VASSAL-3.5.0-SNAPSHOT-db7ecdc7e to see if it fixed another bug, I discovered some really strange behavior; the toolbar with the zoom controls is missing from many (but not all) of my windows. Then, while I had the module open for editing, I opened the definition for one of these windows (by double-clicking the [Map Window] entry), then closed it again with the Cancel button, which caused the window itself to also close, and now the button that is supposed to toggle the window open and closed does nothing...the window is permanently closed!
Ah, figured out why only some of them lost the toolbars; every window that's defined as a [Map Window] has lost its toolbar and refuses to reopen after viewing the main properties; those defined as [Player Hand] or [Private Window] still work properly.
I have not gone back and tried in 3.5.0-beta 1, sorry.