I stumbled across this when i tried to add an ASB as a tooltip to a subapp that i resize at start of frame by the windows control. I wanted to add it at the ground. But surprisingly there was a 5-6 pixel height blank area a the ground then where my tooltip ASB should be.
It turned out it has to do with the settings of the Subapp. With toolcaption checked i get this 5-6 pixel height blank area at the bottom. Without not:
File to have a look at is attached, 12 kb