I wasn’t aware that this problem persisted in a maximized Standalone Game. I’ll update my report and try to escalate it further.
After some tests: I’m running two monitors here, and I suspect that that’s what is letting me escape the window in Standalone Game. I can’t click on the taskbar or any part of Monitor #1 (where the maximized Standalone Game is), but I can move the mouse over to Monitor #2 where I can live-click on other windows, taking focus from the game. Is that what you’re experiencing, too?
Presumably, maximized Standalone Game properly constrains the cursor with one monitor.