1.
Note how the in-game scoreboard sometimes is very late to appear.
As if the "tab"-keypress takes a long time to reach the game.
2.
Note how when the mouse is over the in-game scoreboard when releasing Tab, the in-game scoreboard does not disappear.
As if the "tab"-release is not sent to the game.
3.
Note how, when taking mouse-control during a mouse-less state, the user loses control over the game entirely when releasing Tab.
Keyboard control is also lost, not just mouse-control.
They are forced to Alt-Tab out and back in to regain control.