With the latest launcher update, opening Minecraft now opens the game window behind the title bar, stretching the display slightly to compensate. Because of this, the game thinks its menu buttons, etc. are in different places, causing an annoying de-alignment.

The first screenshot shows the version number and copyright getting cut off by the bottom parts of the menu; I mouse over the "Minecraft Realms" button to demonstrate the de-alignment issue. The second shows the same issue happening in Minecraft 1.8.1, to demonstrate that this issue is not exclusive to 1.12.2, and thus a result of the launcher.