Mouse leaving gameplay

Mouse leaves confine of gameplay tab when not entirely full-screen. Causes random minimizing of tab, opening of task bar, sporadic movements and placing of blocks. Difficult to do anything at all without tab closing suddenly or another opening. Leaves the top of gameplay tab, to minimize or edit or close, and to the bottom, opening windows and task bar. Practically constant interruption now that mouse can leave the confines of the gameplay. Did not do this before.