"The biggest benefit you'll experience is that fullscreen mode now defaults to borderless windowed, so that snapping in and out of fullscreen mode is very quick!" https://minecraft.net/en-us/article/minecraft-snapshot-17w43a

The new borderless fullscreen feature is not actually borderless, it's just regular fullscreen. This results in delayed window switching (in fact, it's even slower than it was in 1.12.2), and the inability to screenshot with PrtSc.

In fullscreen mode, there is no way to take a screenshot without using the game's own screenshot functionality (F2). The PrtSc key simply copies a black image to the clipboard, rather than an image of what's shown in-game.

Steps to reproduce:

1. Open any application in a small window mode
2. Startup Minecraft in fullscreen
3. Use alt+tab to switch to that other application in small window
4. Notice that instead of that application appearing on top of Minecraft window you-re switching to that window