Expensive mounting

[RichardHoekstra]

What is the recommended strategy to deal with components that are expensive (aka slow) to remount? Is there a way to keep the state of the 'offscreen' items in memory, or, the first ~10 invisible items at the top and the first ~10 invisible items at the bottom? I'm having trouble figuring out how to do this. I'm not sure whether I can do it without making any changes to the VirtualList.svelte component.

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[gitbreaker222]

...like a padding of eagerly loaded items, to prevent something like large images scrolling into view to appear after a delay? From my perspective this is not possible with the current implementation (I've already worked with the code for #43)

Maybe this issue can be labeled with "feature request" to have something like

<!--
New property "bleedItems"
Takes a number to render items outside of view. Default is 0.

Name inspired from print, where an image should be larger than final page size 
to prevent flash of white paper due to non-perfect trimming
-->
<VirtualList
  items={items}
  let:item
  bind:start
  bind:end
  bleedItems={2}
  >
  <ExpensiveComponent />
</VirtualList>

[RichardHoekstra]

That'd be what I am looking for yes. I've tried working with the code as well, but, I'm having trouble wrangling it into a form where it does what I want.

[Amerlander]

My issue https://github.com/sveltejs/svelte-virtual-list/issues/56 is a bit different, but maybe also helpfull to yours.

My solution was not to prerender more items, but to wait until the scroll stops and then run the expensive code. In that way if you scroll your way down not every item will run its code, only the ones which are visible when you stop scrolling.