Just a bit ago, I discovered that one can “cheat” the stacking mechanic to make any items stack together, as long as you have an initial stack.
Here’s some evidence:
The way to reproduce this is by having a smart-stack, so for example, multiple stacks of different leaves, and then left-clicking one of them, and swapping with the item that shouldn’t stack, therefore putting it in the smart stack by swapping with the leaf item, for example.
At least I presume this behaviour is not by design, although I have to admit more inventory space would be nice. I discovered it by accident when trying to take something out of a smart-stack to put in a crafting space, but accidentally swapped it out with another item, and I decided to see if it was a consistent behaviour, and it seems to be (as you can see, axe, furnace, leaves, etc all stacked in one smart stack.)