In the 1.13 pre-6 update, I launched the version successfully and played in a few of my worlds. Afterwards, I put my computer to sleep without closing Minecraft. About 10 minutes later I reopened my computer to find the entire bottom-left corner hidden like a missing texture.

I tried minimising the tab, changing between window and fullscreen mode, reloading chunks, opening different things like options and different worlds. This did not fix the problem at all. I did note the pink quarter always stayed the same size, no matter the size of the window, and always perfectly aligned with the bottom left corner of Minecraft.

Finally, I closed the tab and relaunched Minecraft and the issue was fixed.

This effect only occurred in this tab, not in my other tabs which I had open which included Minecraft 1.12.2.

This may be just an anomaly but if it isn't it would be great to get the issue fixed before the full 1.13 version comes out. I have only tried to recreate this bug once and my attempt was unsuccessful in reproducing the bug.

Thanks, vagga2U