Whenever you hover over an item, a tab with the items properties shows up (it is especially big with Advanced tooltips turned on). If the tab is big enough and the item next to it has a larger quantity than one, the number is displayed OVER the tab, not under it.

What I expected to happen was...:
The number of items in neighboring slots would not be visible.

What actually happened was...:
It is visible.

Steps to Reproduce:
1. Open a new Minecraft world in 20w21a.
2. Enable advanced tooltips, then take a random item and place it into a slot in your inventory.
3. Take another, stackable item, and place it (stacked) next to the first item (below or to the right).

4. Hover with your mouse over the item No. 1. The number of the other item is shown.