Once i spawn i can move but then when i stop i cant move, all i can do is look around and place

Once i spawn and run around, once i stop running i cant move at all. All i can do is look around and place.

What I expected to happen was...:
When i spawned i thought i could play minecraft normal with no glitches but there may be a bug.

What actually happened was...:
After i spawned i ran around and once i stopped i couldn't move or choose any of my items.

Steps to Reproduce:
1. Use an apple computer and go to minecraft.net.
2. Click on the download link.
3. Click run at the bottom of your screen.
4. Log In
5. Click single player and create a new world.
6.Once you spawn run around and then stop
7. After this i cant move.