Clicking on X in creative mode makes all inventory and armor disappear forever

Playing in Realms, I was in creative mode with a full inventory and armor. I grabbed something new from creative inventory, but couldn't put it in my inventory because it was full, so I put it on the X to destroy it. Instantly, my inventory and armor were blank! Permanently.

We hypothesize that this issue may be related to a long term issue of sticking shift keys, but it is unclear what would lead to the sticking shift key issue in this case, especially as I was in full screen mode.

I lost things I could not reproduce, even in creative mode, so I don't want this to happen to anyone else. This was worse than almost any death.