Leaving the world softlocks

When trying to exit build mode, you're left on an unresponsive black screen. Don't know why this would be happening, as it wasn't a problem last night when I first tested.

(In vid, trying to click/drag on anything, mashing keyboard, still nothing happens)

https://github.com/user-attachments/assets/5ed65ff8-c952-4f01-a7cd-a7e66f9f7c25

Start by reproducing the black, unresponsive screen when exiting build mode, using the linked video as the behavioral reference. Trace the exit-from-build-mode flow and identify where input stops responding; done means leaving build mode returns to a usable world without a softlock.

Written by the indexing model from the issue text.