Blocks Stay Partially Broken

It would work similar to the furnace family, where if you don't have enough fuel and there's still something to smelt, the smelting progress will slowly go back to zero percent.  However for mining, How about we make it so that whenever you hold down the destroy block button, every block that you're in the process of mining, will keep its level of being destroyed if you so happen to mine another block.  Once you let go of the destroy block button, the level of destruction will slowly go back to zero percent.