The Bug
Every non-inventory GUI in the game has a button to close it ("done"), the Advancement one doesn't.
And while it is reachable via buttons in the pause-menu, it's not closable by a button, only "Esc" or "L" (default) can close it again.
This, while inconsistent with other screens, also has the potential for confusion for players. Also, the old achievement screen does feature a button to close it.
Steps to Reproduce
Open up the advancements GUI. ("L" by default).
Attempt to locate a close button within this menu.
Take note as to whether or not the advancement GUI has a close button.
Observed Behavior
The advancement GUI doesn't have a close button.
Expected Behavior
The advancement GUI would have a close button, just like other similar GUIs.
I know I'm not a mod - but this is a suggestion, it really should go in the suggestion forms on reddit.