After starting the game output window, it will slow down considerably or stop working completely and go completely blank when overloaded with text. Interface and text disappear when it becomes blank. This did not occur with the previous launcher.

Tests:

1. Occurs while switching between resource packs when the resource pack has many errors in it that flood the output.

2. Making a single repeating command block print '/say hi' to the chat causes the launcher to become unbearably slow.