Fix Chrome scroll-up with mouse wheel

[NfNitLoop]

In my current implementation of #69, scrolling up in chrome with the mouse wheel results in very odd behavior. (using PageUp or the up arrow does not.)

Seems to be a bug in the way Chrome's smooth scrolling interacts with calls to window.scrollTo. If I turn off smooth scrolling in the browser's settings, the behavior disappears.

I should probably write up a Svelte playground example and submit a bug report to Chrome.

Would love to find a workaround in the meantime.

[NfNitLoop]

The issue seems to be trying to be trying to compensate for adding items above the view port while scrolling is happening. The fix/compromise is to only load items once scrolling has stopped.