Returning to simple pop-up characters panel?
The button in the upper-right corner of Character Viewer was the culprit.  If I summon Character Viewer and click it, Character Viewer shrinks:



From then on, further presses of control+command+space summon the pop-up panel instead.

As to how I got to the full Character Viewer, opening Character Viewer from the menu bar Input menu (which can be turned on in System Preferences > Keyboard > Input Sources):



then clicking the button in the upper-right corner of Character Viewer twice—once to shrink the window, once to restore it—appears to set a preference that I'd like the key combination to pop up the full Character Viewer instead of the pop-up.