Client sometimes resets chat bar when typing in 'a'

When you type "/help a" in the Chatbar the chatbar reset to "a". When you type in any other letter it doesn't reset.

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^{Edit by [Mojang] Mega_Spud (Jay Wells):}
Example Steps to Reproduce:

1. Start the game in 1.2.6.2 Retail and open a world
2. Open the chat bar
3. Press CTRL+Esc (focus moves from Minecraft)
4. Click focus back on Minecraft
5. Type some text or a command in the chat bar or inventory search