ToseRedstone

For everyone who got this issue, I recommend this AutoHotKey program: https://pastebin.com/sxu0Uaaf

I've been using it for a while now and it really help a lot. It doesn't work perfectly well, sometimes the chat still clears when pressing a, but that's probably only 1% of the time. This small script made Minecraft playable for me again.

Fixed in 18w32a

It's fixed in the latest snapshot

Seems to be fixed in 18w30a

Seems to be fixed in 18w30a

I'm also using Windows 10

Confirmed for 1.13 pre6

Confirmed for 1.13 pre5

Confirmed for 1.13 pre5

The reason your problem happens is that the behavior of observers changed in prerelease 4. Up until 1.13 pre4, observers which detect a 4 gametick signal (equivalent to 2 ticks on a repeater) output a 2 gametick signal, meaning a sticky piston activated by the output signal would drop its block.
In pre 4, this behavior got changed. Observers which detect a 4 gametick signal now output a 4 gametick signal, meaning a piston wouldn't drop its block anymore.

This is a fundamental change in the behavior of observers and would cause many redstone contraptions to break in 1.13 if this bug doesn't get fixed before the official 1.13 release.

Question to the mods: Should I create a new bug report which describes the issue in a better way? I'm asking because this bug report doesn't describe the reason why the redstone contraption broke and it would maybe be a good idea to create a new bug report which is only about the change in the behavior of observers.

Still happens in pre4

I have the same issue too and I noticed that quickly spinning your mouse while holding AltGr is a more consistent way (at least for me) to encounter this problem.
My theory is that this problem is caused by having a small lag spike while holding AltGr. I play Minecraft on a pretty bad laptop, thus changing my field of view very rapidly is already enough to cause some lag.
Another sign that lag could be cause of this issue is that in a void world where my game obviously isn't lagging, I can hold the AltGr-key for a minute without Ctrl getting stuck. But when I quickly move my mouse and thus cause a small amount of lag, the Ctrl-key is stuck only after a few seconds of moving the mouse.
Furthermore, intentionally lagging my game with some redstone contraptions made this issue even more easily reproducible.

I can also confirm that this issue never happened in 1.12 and started happening in the first 1.13 snapshot (17w43a).

I use the German keyboard layout.

I was able to reproduce it without moving the mouse, but reproducing the problem with moving the mouse was easier for me.
You can mark this post as a duplicate and I will add some information in the comments of the other bug report.