bug report: using wireless keyboard, pressing up many times beyond top of document appears to maintain weird state

How to reproduce. Create a JS file. First line should be:

// hello world this is a test

and the second line should be

// this is only a test

Position the cursor on the word "world". Press the Up key three times. Now press Down. Note that the cursor stays on the first line. Also note that if you press the Down key exactly three more times, it takes on the third time.

I'm using the apple wireless keyboard to see this. This bug does not appear to affect the virtual keyboard.