The bug

The Minecraft Launcher's account page does not shorten long email addresses. This causes very long email addresses to actually push the options button to the right on that line, as you can see in the image (below, in the Images section).

Plausibility

Unlike MCL-16558 which is only possible by modifying the launcher_accounts.json file, this issue may actually be possible for a user to experience.

Upon looking into the reason why MCL-16558 was resolved, I tested a long email address instead of a long username. However, reporting the issue without doing further research will cause it to either be resolved as a duplicate of 16558 or Won't Fix. (This may be WF, but I think it's better to report it anyway? Is it?)

According to the Internet Engineering Task Force (IETF), an internet standards organization, RFC-5321 states that a local-path (username) can be up to 64 octets (characters), and the domain can be up to 255 octets (characters). The format used is local-path@domain (the @ symbol is not included in either of these limits), and so using these values, we get a maximum character count of 320. This limit can be exceeded using certain loopholes, but I will not get into those for the simplicity of this report.

320 characters is not very reasonable. How often do you see a domain name that is 255 characters long? You don't. Because each label can only be 63 characters long. A label is a piece of a domain. For example, www, example, and com are all individual labels in a domain.

After searching through various different domain vendors, I found that name.com sells the domain:

hellothisisaverylongdomainnameandiamgoingtobuyitforthemclissues.com

This domain is 67 characters long. Although these words may seem quite unreasonable, something like this is a bit more reasonable, and is also available for purchase:

RanchoSantaMargaritaCaliforniaFamousFlowerSupplyCorporation.digital 

That's also 67 characters long.

And according to RFC-5321, a username can be up to 64 characters long. However, again, this is not very reasonable, as if you had a domain that long, you wouldn't want someone to need to type over 100 characters to be able to send an email to you. So, after a bit of searching around again, I found that the typical limit for the username is 30 characters. So, for the first domain name, I came up with:

myminecraftlaunchertestaccount

For the flower example, I came up with:

ChristopherAlexandersonFlorist

After adding everything up for each example, we get the following two email addresses:

myminecraftlaunchertestaccount@hellothisisaverylongdomainnameandiamgoingtobuyitforthemclissues.com

ChristopherAlexandersonFlorist@RanchoSantaMargaritaCaliforniaFamousFlowerSupplyCorporation.digital

All of these emails are completely reasonable, and there are likely hundreds, or even thousands, of domains this long.

How to reproduce

1. Close the Minecraft Launcher
2. Navigate to your minecraft folder -> open launcher_accounts.json
3. Change the username value of a Mojang account to one of the email addresses shown above
4. Open the Minecraft Launcher with your internet connection disabled (to prevent the email from being reverted back to what it should be)
5. Navigate to Settings -> Accounts
   Notice that the options button is pushed further to the right than normal, and the email address is not shortened

Remember, this may be possible without modifying the launcher_accounts.json file.

Images