(if it's on same button as place block)

Would help in 2 cases:

1. When there are a lot of different colored blocks, like in pixel-art, or just a lot of different blocks nearby;
2. When items in a hand run out:

https://youtu.be/oIbnW8026CI

Based on the video I assume you mean mostly just to avoid placing the wrong block if you hold down the use key instead of clicking separately for every block?

I recently (in 1.12.2) already changed how/where the pick block happens in code. Previously it only happened from actual keyboard or mouse input, when clicking the use key. So it would not happen if you just hold down the key and move to the next block. Now it happens from the item use code, so it happens before every placed block, if the new pickBlockAuto option is enabled. Without this option it still only happens from the bound keybind presses, so usually from right clicking.

I also improved and rewrote the Easy Place mode and the Placement Restriction mode to be a lot less error spammy, so now when you combine pickBlockAuto with the Placement Restriction mode, it will give you pretty nice automatic pick blocking without having to click separately all the time, and it also prevents placing the wrong block when you run out of an item.

So these will be coming to the other versions at some point with the next merge from 1.12.2. I think I should mainly finish the planned malilib refactors/changes/additions and then I should do the next merge, as there are already tons of improvements and additions waiting in the 1.12.2 branch compared to 1.13.2+ branches. Trying to finish the entire to-do list for 1.0 before the next merge would probably take way too long...