Playing in creative mode (debug mode also on). I didn't mean to, but I pressed the Z-key and a bright blue frame appeared. Thinking it may be a toggle, I pressed Z-key again, and the frame doubled in size. It wouldn't go away until after exiting the game. What is that used for?
That's the selection box for building. Pressing Z selects what you're targeting with that blue box. If you press Z again while pointing at another location the selection will include everything from the original selection to your next selection. It can be a useful tool in creative if you just want to have fun building. You can use G to drag the selection, CTRL+G to resize, Shift+G to select an existing block, etc. There are many other controls I won't get into here. If you'd like to learn more about using this to build there are some good tutorials out there like this one: