After the update that has happened around May 7, 2021, the output log is now buggy.
What I expected to happen:
Output log would look normal with white text and would cover the full window.
What actually happened:
Output log was in black (almost unreadable) text, and only covers about one third of the screen.
Reproduction:
Update to the latest version of Minecraft launcher
Start either 1.16.5 or 21w18a with output log on.
Hello! This appears to be a duplicate of MCL-17432 and MCL-17433, which are both fixed for the latest beta version of the launcher.
To enable the beta, select Settings (bottom left corner of the launcher), enable the first check box, and then close and reopen your Minecraft Launcher.