STR:

1. Create a new creative world
2. Add a mob spawn egg to your hot bar
3. Spawn mob on a block

EXPECTED OUTCOME: Mob should spawn on the block (right-click behaviour)
OBSERVED OUTCOME: Block breaks (left-click behaviour)

This behaviour is problematic if you are using only a trackpad but it is replicable with a mouse as well.

Tested as working in 1.12.2.