I alt-tabbed out of minecraft to look something up about one of the new items when i came back to the game the screen was frozen but i could still unpause and move around and break blocks
To recreate:
join a minecraft world
alt-tab out of the game
open minecraft back up
Jumping on this bug before it gets marked as duplicate, there is currently no valid root bug for this particular issue. MC-123498, which currently has a large number of bugs marked as duplicates, is marked as invalid because "this is not a technical support forum".
The 'bug' appears to have shown up in 1.12 as well as the snapshots as a recent change, and all of the bug reports which include an environment list Windows 10. I can confirm that this bug appears in Windows 10 but not Linux Mint 18.3 while fullscreened in both 1.12.2 and the snapshots.
It seems likely that a change in Windows 10 has caused a problem with how Minecraft draws its window.
EDIT:
To clarify, the bug appears in fullscreened but not windowed mode.