I tried to run modded Minecraft using the new Launcher and advised the Launcher to show me the game output, but it opened just a blank window without content which didn't show anything also after waiting some minutes.
I think the problem is on your side because the game output shows up in the launcher log which i have attached.
MCL-5931
Game Output isn't showing up with modded Minecraft
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Thank you for your report!
However, this issue has been closed as Invalid.
Your game, launcher or server is modified.
If you can reproduce the issue in a vanilla environment, please recreate the issue.
+ Any non-standard client/server/launcher build needs to be taken up with the appropriate team, not Mojang.
+ Any plugin issues need to be addressed to the creator of the plugin or resource pack.
+ If you have problems on large servers, such as The Hive and Hypixel, please contact them first as they run modified server software.
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