Why does the taskbar open when I drive over it in a game?

I wanted to ask why, if I play Minecraft, GTA 5 or Rocket League, for example, the taskbar just reopens when I drag the mouse cursor down. Goes into the games pretty annoying, because then you press the left mouse button and opens a new program.

Actually, the game should take the full screen and be of the authority over Win.

If you fix the taskbar (that does not hide it) does that happen too? And you play the games in full screen or in windowed mode?

I always play in full screen, and I have not fixed the taskbar. I'll try it out now.