Not sure if this only applies to Windows 10 s, as for I changed just last week and this bug/glitch appeared. While I'm paused, to alt+tab out to check something else, in osu! s it as if I'm pressing tab to remove the leader board, even though that's not my intention. As shown below.
osu! version: 20160409cuttingedge
osu! version: 20160409cuttingedge