Pretty self explanatory. When you load a singleplayer world, it should instantly pause the game when loading completes.
I've got nearly 200 mods, loading my save takes a while, and I haven't even been in this world that long (28 hours), so it's not uncommon for me to press load then alt-tab or walk out to the kitchen or something. Then, occasionally, I'll come back to find myself getting attacked by a drifter or starving or something, because the thing I was doing on the side took longer than I expected, or I forgot the game was loading (I'm a dumbass like that), or whatever happens that causes me to not be immediately available when the game finishes loading.
