While in-game, use the system shortcut "Command+Tab" to switch to another running application. Then, return to Minecraft.

It seems like the client thinks you are still holding the Command key.

This is very annoying, and at times embarrassing. Trying to chat will 90% of the time result in spewing some nonsense, as any hit of the "A" key will select all text (which gets erased by the next keypress)- "C", "X", etc will get skipped.

Hammering on the Command key a few times while in chat seems to fix the problem temporarily, but it is difficult to remember to do that every single time I switch away from the game for a moment (checking console on my server being a frequent example)