Status: Cannot reproduce, but I suspect that it will be fixed when the root cause of several bugs is fixed, including:
Can't tab back into game, Windows 7 (http://www.lemmingsforums.net/index.php?topic=2405.0)
Moving window moves only part (http://www.lemmingsforums.net/index.php?topic=2406.0)
Flickering between screens in windowed mode (http://www.lemmingsforums.net/index.php?topic=2398.0) <--- this topic remains open
-when in windowed mode [on windows 7] the game doesn't appear on the tab menu and cannot be tabbed to. You can tab away but not back to the game.
Tabbing works as normal in fullscreen (as it did in previous versions)
I cannot reproduce this problem on Windows 10, with NeoLemmix V1.38n-B. I don't know when I'll have access to a Windows 7 system, but I'll test there when I do.
Most likely, this results from the very weird way Lemmix handles display (this was true even for EricLang's versions of regular Lemmix): The "full screen" mode is actually a maximized window, with no borders, that's completely black; while the game area (be it a menu screen or gameplay) is another window placed over the top of this, also with no borders. This works pretty well for full screen, but is a major pain in the ass when trying to implement a windowed mode. The windowed mode works by adding borders, and hiding the "main" window when not transitioning between other screens (it needs to be shown at this point to avoid having the window seem to disappear then reappear).
I intend to replace that with a more-normal setup at some point; I would think that once I do so, it'd fix this bug.
EDIT: Could you expand on this a bit:
1. Does it happen always when in windowed mode, or only on certain screens?
2. If when you start NeoLemmix the setting is windowed, and you later change to fullscreen (without exiting), does the issue continue to happen?
3. What about the reverse - if you start fullscreen, then change to windowed, does this happen?
1. always window, doesn't matter which screen
2. no it starts working fine when you change to full screen
3. yes, it still happens.
I just noticed something else [again very minor]; if you tab away during a loading screen the screen remains blank forever until you tab back to it. [where as normally the game continues while in the background]
Just checked it on my computer with Windows 7 and I cannot reproduce this bug. With windowed mode (3x Zoom) NeoLemmix appears on every tab menu I can think of and can be selected.
Found two other new bugs though...
Also got Win 7 and nope. Everything fine here either.