Mounting a horse and opening your inventory opens player inventory

Mounting a horse and opening your inventory immediately after opens the player's inventory and mounts the horse.

What I expected to happen was....

Mount the horse and open it's inventory

What actually happened was....

I mounted the horse but still opened my inventory

Steps to reproduce:

1. Have a saddled horse
2. Right click to mount it
3. Open your inventory as soon as you do
4. → With good timing, the player's inventory opens instead of the horse's