Go to any ROBLOX game in Play mode, enter first person, alt+tab out, and without selecting the window, move your mouse over it. Nothing happens – this is good behavior.
Now, go into Play Solo in an empty baseplate in Studio, enter first person, and alt+tab out. Mouse over the viewport and it steals your mouse, locking it to the center of the viewport. This gets pretty annoying when I’m trying to get between widgets in Studio and accidentally pass through the viewport and my mouse locks up. It gets even worse on occasion when alt+tab fails to free the mouse and I’m stuck in Studio until spamming alt+tab finally frees me.