Block breaking is not canceled when losing focus of a block

How to reproduce

1. Look at a block and start mining it
2. Walk / look away from the block so you lose focus of it while still holding left click - the cracks disappear, working as intended so far
3. Without letting go of the mouse button, walk up to/look at it again so it is focused - cracks start to appear again, working well
4. Walk / look away again - the cracks stay, there's your issue

See also MC-125127.