After a recent update to the UI Editor (probably within the last week or so), you can no longer use the arrow keys to navigate up and down the Explorer when you have a GUI-related object selected. Instead, the arrow keys will just move the camera instead. The arrow keys work correctly when you have a non-GUI object selected, though.
- Open a place and add a ScreenGui into the StarterGui
- Select the StarterGui (or the ScreenGui) in the Explorer
- Press the arrow keys. Instead of changing the selected object, this will move your camera instead.
Deleting the UI Editor from your built in plugins stops this issue from happening.