Up and down shouldn't scroll the page

[pinobatch]

Environment: Firefox 78.6.0esr (64-bit) on Debian 10 on a ThinkPad X61 tablet with a 1024x768-pixel internal display

Steps to reproduce:

1. Resize the window to be small enough to have a vertical scroll bar. (The X61 display is small enough that this happens by default.)
2. Open any version of 240p Test Suite for NES.
3. Click the emulated screen to give it focus.
4. Navigate the menus.

Result: Arrow keys navigate the menus while also scrolling the page.

Expected: While the screen has focus, arrow keys go only to the