fix(scroll-assist): improve input scroll accuracy with native resizing

[liamdebeasi]

Issue number: Part of #22940

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What is the current behavior?

While working on a fix for #22940, I discovered another bug that impacted the reliability of my proposed fix for #22940. When we compute the scroll data (i.e. how much the input needs to be scrolled by), we subtract the keyboardHeight from the platformHeight (i.e. the viewport height):

ionic-framework/core/src/utils/input-shims/hacks/scroll-data.ts

Line 34 in 1015c06

const visibleAreaBottom = Math.min(contentRect.bottom, platformHeight - keyboardHeight);

Every time we tap between inputs (even if the keyboard is already open) we re-run scroll assist because the newly focused input could be partially obscured by the keyboard. However, in this case we scroll by too much because we effectively subtract the keyboard height twice. This is because by the time we compute platformHeight, the platform dimensions have already shrunk to account for the keyboard (when the webview resizes).

As a result, when we subtract keyboardHeight we get a visible area that is much smaller than the actual visible area.

Examples below with different resize modes. Notice that with the "Native" resize mode (entire webview resizes when keyboard is open) tapping into other inputs scrolls the content by much more than it needs to.

Body	Native	Ionic	None
before-body.MP4	before-native.MP4	before-ionic.MP4	before-none.MP4

What is the new behavior?

• We now compute the viewport height on load rather than on focus. This ensures that we always use the full viewport height when computing the visible area.

Body	Native	Ionic	None
after-body.MP4	after-native.MP4	after-ionic.MP4	after-none.MP4

Does this introduce a breaking change?

• Yes
• No

Other information

Dev build: 7.3.4-dev.11694548895.1578981b

[liamdebeasi]

ownerDocument gives you the document for a node: https://developer.mozilla.org/en-US/docs/Web/API/Node/ownerDocument

defaultView gives you the window associated with the document if available: https://developer.mozilla.org/en-US/docs/Web/API/Document/defaultView

I opted to simplify this by using our win util since that accounts for SSR: https://github.com/ionic-team/ionic-framework/pull/28169/files#diff-ebd08eae5669c5c10420cdb5d258ee6db362b7796cc76ee15e34441a84d48888R51