ShadowDragon

No. My problem is, that you can place nether portals in the nether at locations that cause a portal to spawn behind the world border in the overworld. This will cause you to immediately die.

E.g. the world border is set to 8999 (overworld). Then you place a portal at X=0 and y=2000 (in the nether) and a new portal is created at let's say y=16000 (overworld). The moment you go through the portal in the nether, you'll die as you are some 7.000 blocks behind the world border.  This also means you will lose all your items and experience you had.

Thought this could be fixed by adjusting the world border for all dimensions accordingly. (Change the world border in 1 dimensions and it adjusts the world borders in all dimensions, but by taking the coordinate ratio between those dimensions into account.)

Another possible fix would be to just disable all portals which would lead to a location behind the world border.

It's still a problem in 1.15.2.

Minecraft 1.14.4 Pre-Release 2 is still affected (I can't edit the affected versions on mobile).

It's still a problem on Minecraft 1.14.3 Pre-Release 3.

I could reproduce the problem on Minecraft 1.13.2-pre2, even thought this Bot thinks the version hasn't been released yet. My Minecraft Launcher tells me it has been released.

I could also reproduce this on Minecraft 1.13.2-pre1.

I've also found out, the issue also applies to the normal backup functionality of Minecraft (see the updated description of the issue) and worlds as small as 460MB on my SSD also cause the issue to occur, so no need for a multiply gigabyte world to force this issue to occur (at least on my system).

Ok, I just added the launcher log to the issue (after it appeared again. After the backup was done and thus the world upgrade happens I clicked cancel as the world upgrade itself isn't affected).

Afterwards I tested it again with the same world and it happened again (look at screenshot-1 for details). For this test I also decided to have the log output window open to better see what the log put's out when it occurs and afterwards.

Also note that whether this issue occurs depends on the world size. Smaller worlds aren't affected, thought I haven't figured out the smallest necessary world size for the issue too occur, thus I'm always going with the largest map I've got as I know that it will definitly happen on this map size.

Yeah, but converting multiply chunks at the same time (I mean, I've got 12 threads) would greatly improve speed.

java -jar minecraft_server.jar nogui

Using the version without an extra GUI may also help improve performance, thought I'm not sure if this is the cause of the problem you are experiencing.

I could also reproduce this issue with Minecraft 1.13-pre5 running:

Windows 10 (x64, 1709) with 32GB RAM and an i7 6800k processor + AMD RX Vega 64 graphics card (driver: 18.6.1).

Minecraft Launcher: 2.1.1216 (with Java Version 8 Update 51)

JVM arguments: -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M

Everything else in the launcher is at default. View distance was set to 32. For me it took about 20 to 30 thousand blocks for the issue to appear while sprint flying through a newly generated world. I tested it three times and could always reproduce this issue (thought it never crashed for me).