[FF] PageUp/Down doesn't scroll

[Garry Yao]

1. Load the attached sample page;
2. Just put cursor in editor without any other keystrokes;
3. Press PageDown keys couple of times then PageUp;

• Actual: Cursor moves in the document, while content not scrolls;
• Expected: Cursor moves with page scrolls cursor into view.

In addition, note the behavior will return to normal once Down Up/Arrow key is used.

[Jakub Ś]

Reproducible from rev [7369]

[Garry Yao]

Fixed with [7431].