Keyboard navigation (page down/key down) get stuck after a while

[vitalybe]

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Reproduction:

1. Visit http://demo.stackfull.com/virtual-scroll/#/biglist
2. Focus on the text
3. Hold "Page down"
4. It will page down several times but will stop after a bit

Scrolling with a mouse makes page-down work again for a while, until it stops again.