PageDown isn't handled properly in slash command typeahead.

[Ovgodd]

Describe the bug

When the slash command typeahead menu is open (triggered by /), pressing PageDown scrolls the whole page instead of scrolling/paging within the typeahead list itself.
Arrow up/down correctly move the selection inside the menu, but PageDown is not captured by the menu and is handled by the page, which is confusing and breaks the expected keyboard navigation for a suggestion list.

To Reproduce

1. Type / to open the slash command typeahead menu.
2. Ensure the menu is visible and that you can move between items with the up/down arrow keys.
3. Press PageDown.
4. Observe that the browser page scrolls instead of paging/scrolling the slash menu.