When you are normally playing the game (i.e. not in the menu) and press alt-tab to switch to the next window, the game will open the menu with a short delay and it will reset the mouse pointer to the center of the Minecraft window.

This is in windowed mode. And of only of influence if you alt-tab quickly, to the next window. If you slowly alt-tab, searching for the right window, the game will have loaded the menu already. But it also happens when you want to use your mouse to select the window in the alt-tab 'popover'

Desired behavior: Minecraft should not meddle with my mouse pointer at that point.