If you have a message in the chat, there is a background behind it. When it fades away after a few seconds, the text stays at 100% opacity while the background fades out to 0% opacity. Then, after the background is gone, the text instantly disappears.
Environment
This is on Mac OS X with all of the default Minecraft 13w39b multiplayer settings and Java 7 (maybe 6?) in fullscreen mode.
Linked issues
relates to 1
Attachments
Comments 2
MC-36812 is a regression of this.
Can confirm.