perf(picker): avoid flicker on ios

[liamdebeasi]

Issue number: N/A (see below)

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What is the current behavior?

Mike found an issue while testing the Ionic 8 beta where there's a flicker when presenting a picker in a modal on iOS. We've seen this issue before, and the reason why it's happening is there's a quirk in WebKit where the IntersectionObserver callback is fired after an accelerated animation completes given a particular IO configuration.

The end result is there's a delay before each picker column is scrolled to the correct place.

In particular, the modal enter animation on iOS is an accelerated animation, and we use an IO to control when the picker columns should scroll their active options into view.

What is the new behavior?

• The root of the intersection observer is now the parent picker element which avoids the WebKit quirk.

feature-8.0	branch
before.mov	after.mov

Does this introduce a breaking change?

• Yes
• No

Other information

Dev build: 8.0.0-dev.11709226349.179192de

Note: Given that this bug is due to a WebKit quirk and is timing-related, I did not add a test.

[thetaPC]

LGTM, it works great! The flicker is no longer there.