When I started Minecraft in Version 1.8.9 today, the Game Output opened as I wanted it, but the area where the actual output was shown is too small for you to read anything good and the color codes at the beginning (timestamp and where the output is from) are missing.
In the last version of the launcher, this still worked.
Hello! This issue appears to duplicate MCL-17432 and MCL-17433, which are both fixed in the latest beta version. To enable the beta version of the Launcher, go to the launcher's Settings page, and check the first box.