I reopen an issue (MC-79362) which was closed, but the bug stills present. In the chat, alt combinaisons don't work. For example, to make a 'É', we must do the combinaison Alt+144, but we can't.
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If I can help you, the problem appears in typing : copy / paste text with special characters (like É) works, the packet sending / receiving works too. This is when the keys are typped that the problem appears.

Is this still an issue in the current Minecraft Snapshot 15w47c or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Confirmed in 15w47c

Is this still an issue in the most recent versions (i.e. 1.10.2, or 16w42a) of Minecraft?

Fixed as of 1.13-pre5

Presumably a LWJGL3 fix; I'll try to confirm that...
I don't you are supposed to be able to type ALT codes into chat. It's like renaming items in an anvil, or typing something in the GUI of a command block.