fix for the ComboBox positioning issue

[kaizjen]

This PR modifies how <ComboBox> calculates its offset.

It uses el.getBoundingClientRect() to detect the difference between the selected item and the ComboBox's button and moves the menu by this difference.

This does not fix the other issue when the ComboBox will be displayed out of the screen's bounds. I left a proposed solution commented out, because I think it's not the most optimal way of doing it.

Doesn't yet close #6, because some display issues still remain.

[vercel]

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Name	Status	Preview	Updated
fluent-svelte	✅ Ready (Inspect)	Visit Preview	May 29, 2022 at 4:16PM (UTC)

[kaizjen]

Note that to compare how my solution works with the current ComboBox implementation, you will need to create more than 14 items in the ComboBox. I didn't want to edit the 'test' page, so I decided not to include it in the commit

[Tropix126]

Thank you! LGTM.

[kaizjen]

There's an interesting issue with my implementation: it doesn't work with scroll-behavior: smooth. It seems like it's records the selected item's position before it was actually scrolled to.

To fix this, we just need to put scroll-behavior: auto on the <ul> element. I guess I should make another PR for this, since this one was already merged?