This still applies is 0.16.2. I went and found a new fortress spawning wither skeletons the first time, got my wither skulls. I died on the way back to the portal and lost everything. I went back to the fortress and now no wither skeletons spawn. Please fix this ASAP mojang.
Experiencing this issue too on iPad 5th Generation. This change is extremely irritating because now I cannot differentiate enchanted book or split stacks. Please revert to original drag and drop system as these new changes haven’t improved the Classic UI at all.
Edit 17/04/18 - The tooltips now display again as of version 1.2.15 (update is iOS only). However, this fix does not restore full functionality before 1.2.13, as tooltip now blocks items if they are selected in the bottom left corner of the hot bar. Please just restore the old system where the item would ‘float’ above the tooltip and you could drag it anywhere on the screen. The new system is highly annoying as it requires extra taps to use and is difficult to split stacks.