This one seems to be a common issue that people with children and pets encounter; a button-mashing session between said small being and the keyboard can sometimes leave your browser moving a cursor around the webpage when you press the arrow keys, instead of the keys scrolling the webpage up and down. Fortunately it’s an easy fix – it’s a selectable mode called caret browsing mode. Caret is another name for the cursor you see!
To enter or exit caret browsing mode, hit F7. That should resolve this issue straight away.