Of what I found, in 1.15-1.15.2 the upper right corner of the inventory locks after getting into the world, the only way it doesn't lock is when inside of a chest. The cursor doesn't at all pick up the items either when in that corner, the 1.16 pre-releases work, but if possible could this be fixed, maybe as one of those miniupdates before 1.16 comes?
See MC-143776